High School Mathematics-Physics SMILE Meeting 1997-2006 Academic Years Optics

16 September 1997: Pete Insley [Retired from Whitney Young HS; now at Columbia College]
How many images are produced by an object between two mirrors?

Two mirrors made of mirror tiles (approx \$1.00 each at Home Depot; etc) are taped together and set at various angles. How many images are seen?

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TOP VIEW
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 angle #Images 90o 4 60o 6 45o 8 30o 12

Students were asked to draw angles with a protractor, and the mirrors were then set on the students sketches; the class was asked to tell what number of images they saw.

Next we were asked to predict what was the angle for 5 images....First 75o was predicted, and then someone came up with 72o. She said that the product of angle and number of images would be 360o ... Thus 360o/5 = 72o .......

Predict .. then .. Test

Next Peter pulled out several 2x2 squares. He put the 3 squares in the space between the mirrors set at 90 degrees [90o].

 Area Perimeter 12 16

Area = 12
Perimeter = 16

Area = 12
Perimeter = 16

Area = 12
Perimeter = 20

By changing the positions, the areas of the 3 squares remains the same but the total perimeter can change. He asked for predictions and then showed the answers. He suggested we think about cubes with three mirrors.

16 September 1997: Ann Brandon [Joliet West]
ISPP is vending laser Pointers for \$25.00 With Batteries
Wavelength 670 * 10-9 meters [670 nanometers].

She had a diffraction grating [http://230nsc1.phy-astr.gsu.edu/hbase/phyopt/grating.html] of 13,400 lines per inch
Conversion to metric:

1 inch/13,400 * 2.54/1 inch * 1m/100 cm = 1.894*10-6 meters

l = d*x / angle * N {number of diffraction peak) = 1.894*10-6 * 2.25 / 6 * 1 =>710 *10-6 meters

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14 October 1997: Larry Alofs [on leave from Kenwood High School]
He reported on complications of his eye problems

Review - It seems that heredity was important in being susceptible to what seemed at first a detached retina [http://en.wikipedia.org/wiki/Retinal_detachment].  Laser and cold compresses(?) were used to re-attach it, but there still seemed to be stress on the cornea. To get pressure in the attachment region, a band was placed around the eye to get the vitreous humor to push back the retina. Lately he noticed a distortion in the center area of the eye. Seems that some renegade muscular fiber was growing and causing the problem. They replaced the vitreous humor with Silicone Oil and removed the lens to be able to get and clear the growth. For a period the Vitreous Humor has been replaced by a salt water solution. Care must be used because the solution between the iris (where the lens was removed) and the cornea cannot be allowed to mix with the solution in the eye. Without the natural lens he found that a 7 diopter lens in front would allow some focus. He must keep his head straight forward to down. ...Never look up. He looked at an old Physics Book to learn that a diopter [http://en.wikipedia.org/wiki/Diopter] is the inverse of the focal length [given in meters]. [Note: diopter units are used by ophthalmologists because lenses in diopters can be added, whereas we poor physics teachers have to add inverse focal lengths; etc.

1/f = 1/f1 + 1/f2 versus d = d1 + d2

Even negative lenses can then be tested, and also the vari-opter uses a set of lenses that are added to get the desired focal length.]

28 October 1997: Bill Shanks [Joliet Central HS, retired]

Bill brought in a Diamond Industries Light 500 flashlight that was said to be designed by Physics people. The flashlight is square and is said to have no reflectors. A the front surface looks like a type of Fresnel lens, but actually is not that at all. He suspects that total internal reflection from prism surfaces is used to bend the light. The flashlight projects an beam for at least 1/8 mile [250 m]

Small (4 AA cells) \$13 Large (4 D cells) \$20

11 November 1997: Bill Shanks [Joliet Central HS and Joliet Junior College: happy and enviously retired!!]
He showed the effect of heat on a laser beam. Using an old heater he showed how the beam passing above would bend up. The beam projected on the board would rise and be very jumpy when passing through the air above the heater.

```                            \\\\\\
\\\\\\
Wave Front  ||||||||| \\\\\\    first part to enter
>>>>>>    ||||||||||| \\\       the slower medium
||||||||||| \
______________________________________________________
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Also bending of light and sound over cold water were discussed. We can see further (Benton Harbor etc) sometimes because of the cold water even though line geometry says "no". One can hear people across a pond, which normally would not be possible.

FATA Morgana - [named after "Fairy Morgan" of King Arthur's Legends].
In Polar region things look closer than they are; light is focused by passing over the polar ice. See the website http://en.wikipedia.org/wiki/Fata_Morgana_(mirage).

An excellent reference to luminal atmospheric phenomena is the Scientific American reprint series
LIGHT FROM THE SKY [WH FREEMAN 1980] ISBN 0-7167-1222-9. See also http://www.atoptics.co.uk/.

24 March 1998 Larry Alofs [Kenwood High School]
He gave an update on his ocular problems, emphasizing the "physics" in his treatments.

He commented on the use of a storage oscilloscope and a pulse to determine the geometry of the parts of the eye, and thus what focal length lens would be needed. The procedure uses a single pulse in the seen on the "mega-Hertz oscilloscope trigger", which was reflected in the eye at the interface between the salt water and silicone oil [used in the process of replacing the natural fluids in the eye], and also at the back. The velocity of sound in salt water is about 998 m/sec; in silicone oil about 700 m/sec.

From the floor there was a comment that B ultrasound also measured the intensity and the distance of the reflection.

29 September 1998:  Theresa Tobecksen [St Andrews, Calumet City]
She teaches elementary school-4th grade - all subjects

She showed a material tester, which had a flashlight bulb, a pin hole, and a mirror. It showed transmission of light. She showed a periscope. Also she showed a cylinder covered with mylar, where distorted pictures are made recognizable by the reflections from the cylinder. Roy Coleman commented about anamorphic art.

02 February 1999: Ann Brandon [Joliet West HS]
She showed by using a diffraction grating finding the wave length. She passed out a worksheet that used [1] Laser Printer and [2] a diffraction grating with known parameters. She used the grating at 2 meters, measured the diffracted image, and then calculated the wavelength

07 September 1999:  Alex Junievicz [CPS Substitute]
showed one example of a free give-away usual available at ISPP meetings, which displays an infinite set of reflected images of Christmas lights.

14 September 1999: Bill Blunk [Joliet Central HS]
showed us a pair of old tank (as in WW II) periscopes, and then held them sideways up to his eyes, so that he would see things as if his eyes were as far apart (about 0.5 m) as the objectives of the periscopes [http://en.wikipedia.org/wiki/Periscope]. He then asked us how this might affect the way we interpret what we see. After a few guesses, we were invited up to try the experiment for ourselves. Bill stood about ten feet in front of the observer, then extended his arm out toward the observer. Wow! It appeared as if his arm was very long, coming impossibly closer to the observer. Quite an effect!

12 October 1999: Bill Blunk [Joliet Central HS Physics]
"Everybody knows" that the image on the retina of the eye appears inverted [http://library.thinkquest.org/C003776/ingles/images/ojo.gif], but that it is difficult to show it directly. He followed up on a suggestion by Earl Zwicker [IIT retired and still flying low!] to do the following direct and simple experiment: Take a bright, small, well-focused light source [souped-up versions of the Maglite™ are readily available [where, Bill??]. First put the light behind your hand and observe that the reddish component goes through [diffuse reflection]. Then, put the light inside your mouth to show that red light penetrates your cheeks. Finally, put the light on your temple just above the nose, close your eyes, turn it on, and notice the reddish image at the BOTTOM of your field of sensation. Although the light surely hits the TOP of the retina [near where you put the light source!], you "see" it at the bottom. Therefore, your brain must invert the image, and you avoid that upside down feeling, unless of course you live in Australia. Bill passed a CLEAN flashlight around the room so we could see it. In a dark room an ordinary flashlight will suffice.

PJ Note: The "pulse-OX monitor" in ERTM and real hospitals is a little [red] laser source- detector that goes over a finger to measure absorption by oxygenated hemoglobin in blood; the O2 content of blood can be continuously monitored.  For a picture see this. It is important to attach the "pulse-OX monitor" on the different arm from that of the automated "blood pressure cuff".

Bill saw a t-shirt out west with a bear logo and the phrase "send more tourists; the last one tasted delicious". I shall not reproduce the subsequent fascinating discussion of mores of grizzly bears and their interactions with the human environment.

26 October 1999: Earnie Garrison (Jones Commercial HS)
showed us "Pigs in Space." He used a "Mirage" (two concave mirrors facing each other, with hole at the "center" of one for the formation of a real image of an object place at the center of the other, from American Science and Surplus: http://sciplus.com/). A small "pig" object placed inside on the bottom mirror had its real image displayed at the hole of the top mirror. You try to grab the pig, but it isn't there! Bill Blunk's mini-video camera displayed the image on the screen, showing that it is a real image! Isn't it great how one idea works to help another at the same meeting!?

26 October 1999: Bill Blunk (Joliet Central HS)
showed off his mini video camera (Sony NTSC color camera, CCX-Z11) \$90 plus shipping (handout: from All Electronics Corp, 1-800-826-5432, PO Box 567, Van Nuys, CA 91406-0567, http://www.allelectronics.com/, Cat # VC-1100, includes ac power adapter, audio/video cable and instructions). He hooked it up to SMILE's 12 inch monitor and we could see ourselves "up close and personal." Using Quicktime Graphics on a PC, users can see each other! Putting a lens close in front of the camera enabled us to see an upright image; farther away the image became inverted. A bigger TV is more impressive.

Bill also showed us a rocket launcher contrived from paper clip, matches, aluminum foil. All wonderful ideas!

14 March 2000: Shirley Hatcher (Williams School)
showed (handout) us a "Color Analyzer". As an example, she showed us three pages with colored patterns, and cards with three openings covered with different colors of cellophane. Then she gave each of us our own "Secret Message" sheet with a pattern. We used crayons to color each numbered section of the pattern with the color named on the paper: 1 = orange, 2 = green, 3 = yellow, etc. Then we looked at our pattern through the different colored cellophanes. Do you know what we saw? Can you guess? A good way to learn something about how we see colors!

11 April 2000: Karlene Joseph (Lane Tech HS)
gave us each a Count Chocula cereal box. The box had pictures on it, and came with glasses. When we looked at the box through the glasses, the pictures appeared 3D. But then she showed us some colored squares drawn on a paper with neon markers. Using the glasses, we observed that some of the colors seemed to come out in front of other colors, and especially when they overlapped. Green, aqua and pink seemed to stand out more than others. And which ones stood out seemed to depend on the particular combination of colors. Colors seemed to reverse their "standing out" behavior when comparing the neon marker drawings with the cereal box pictures. Much discussion, and speculation on possible wavelength dependence of these phenomena. Fascinating! Can anyone explain this?

27 March 2001 Larry Alofs (Kenwood HS, Physics)
obtained two sheets of polarizing material, along with a quarter-wave plate and a half-wave plate as a give-away at the 13 March 2001 ISPP meeting at the University of Chicago.   However, he wasn't sure how the wave plates should be used, and he brought them to the meeting for experimentation.  We learned that when the two polarizing sheets were placed at 90o to one another, no light could pass through them.  However, when one of the wave plates was placed between the polarizing sheets in that crossed position, light did pass through.  How come?  What is a wave plate?

Comments by Porter Johnson:  Most crystals are optically anisotropic, in that the the velocity of light passing through the crystal depends upon the direction of polarization of the electric field vector E.  Let us consider normal incidence of light [z-direction] upon a thin crystalline material such as mica or quartz, with different speeds for electric field in the x and y crystal axis directions, respectively.  In a quarter wave plate the phase difference for light polarized in the x- and y-directions is 90o, or a quarter wavelength.  Suppose that the light passes first through a polarizer, becoming linearly polarized in the process. Then let the light pass through a quarter wave plate that is tilted at 45o to the direction of the polarized light.  Relative to coordinates in the quarter wave plate, the light is a superposition  of light with electric fields in phase [for each photon, at least] with equal components in the x and y directions.  The quarter wave plate produces a phase difference of 90o for the y-polarized component relative to the x-polarized component, resulting in circularly polarized light.  A schematic diagram is given below:

A

Circularly polarized light is a coherent superposition of the states of linear polarization, with equal weights. Therefore, when we put a second polarizer behind the quarter wave plate, we observe 50% transmission if the second polarizer is parallel to the first, and 50 % transmission if the second is perpendicular to the first.  We should thus observe half intensity for both cases.

In a half-wave polarizer, the relative phase in the crystal axis directions is shifted by 180, so that, in effect, Ex ® Ex and Ey ® - Ey. Thus, if the crystal axis is placed at 45o to the initial direction of polarization, the beam will be polarized at 90o to its initial direction after leaving the crystal.

Here is a description, from that same source, of the effect of a half wave polarizer, in which the phase difference between x-and y-polarized waves is 180o:

"A simple polarization rotator consists of a half wave plate in linear polarized light. Rotating the half wave plate causes the polarization to rotate to twice the angle of the half wave plate's fast axis with the polarization plane, as shown in Figure 5A. We achieve variable polarization rotation by aligning the fast axis of a variable retarder at 45° to the incoming polarization and following this component with a quarter wave retarder with its slow axis aligned with the incoming polarization as seen in Figure 5B. The amount of rotation achieved depends on the amount of retardance exhibited by the first retarder. The polarization axis is rotated to an angle that is one-half the phase shift provided by the variable retarder."

01 May 2001 Fred Farnell (Lane Tech HS, Physics)
reflected a laser beam off a mirror [with silvering on the "back surface" behind a glass layer] and we saw 4 reflected laser "spots" on a screen.  The spots were produced by reflection off the front and back surfaces of the glass layer, as well as multiple reflections, and it was suggested that there might be additional spot images with lower intensity.

Then he reflected the laser off a white plastic surface.  It was hard to see any image on the screen; the "spot" was very diffuse and spread out.

Finally he reflected the laser off the shiny side of a piece of aluminum foil, and the resulting smeared image resembled a pattern from a diffraction grating. In fact, one could see lines in the foil by close examination.  When the foil was turned by 90°, the smeared image was also rotated by that amount.  Upon repeating the experiment with the dull side of the foil, two-dimensional diffraction patterns were found.

Larry Alofs suggested that thin plastic layers are put on both sides of certain types of  aluminum foil, and it was speculated that this might be to reduce oxidation. For more information on Aluminum foil, see the website http://www.aluminum.org/Content/NavigationMenu/The_Industry/-Foil/Foil.htm.

Fred Schaal asked how long it takes copper sheets to form the (green) oxidation layer.  According to Porter Johnson, the time scale is about 18 months, if the copper sheets are exposed to the elements. For details see http://www.copper.org/applications/architecture/finishes.html.

Arlyn Van Ek asked why an index of refraction occurs in a material, when light always travels between the atoms with velocity cPorter Johnson made the following comments.  A freely propagating plane wave of electromagnetic radiation can be written (leaving out its state of polarization) exp[ i k · r - w t] = exp [ i k r cos q - w t], where k is the wave vector [z-direction] and w is the angular frequency. In the presence of a spherical scatterer (a free atom) at the origin, there is also a scattered wave (spherical polar coordinates, with time factor suppressed):

exp[ i k z] ® exp[ i k z] + F(q) exp [i k r] / r

The function F(q) is called the scattering amplitude; in general it is complex and dependent on wave number k. The two terms are equally important for propagation of light in a medium, and we obtain the form

exp[ i n k z].

The index of refraction of the medium of (randomly located) atoms, n, is given in terms of the forward scattering amplitude F(0), and N, the number of atoms per unit volume, by the following formula:

n = 1 + 2p N F(0) / k2 ,

The imaginary part of index of refraction corresponds to attenuation of the beam.  The intensity transmitted to depth z is given by the relation

I(z) = exp[ - 2 k z Im n ] = exp[ - 4p N z Im F(0) / k] = exp[ - N z s] ;

where s = 4p Im F(0) / k is total scattering cross section. This latter relation is known as the optical theorem.

In summary, the index of refraction arises because of the scattered wave, which affects the amount of light passing through the material. For details see Scattering Theory of Waves and Particles by Roger Newton [McGraw-Hill 1966] pp 24-28.

01 May 2001 Don Kanner (Lane Tech HS, Physics)
looked through a magnifying glass (lens) at a pencil that he held on the other side at a distance less than the focal length, f, of the lens. Don then told us how his students were mostly confused about where the image of the pencil would be, and how it would look. He told us that, according to optics, an erect, virtual and magnified image is formed on the same side of the lens as the pencil. How could we see it? By placing a screen at its location? But then he showed us the image by turning the lens-and-pencil so we could look through the lens and see the virtual image for ourselves. We knew it had to be a virtual image since it was larger than the actual pencil as seen without the lens. But some of his students were surprised that they could see only the virtual image through the lens, and not the pencil "itself" --- after all, the lens is transparent and they thought you should be able to see the "actual pencil"! Great ideas, Don

01 May 2001 Marilynn Stone (Lane Tech HS, Physics) Polarization of Light
made a pinhole at the center of an opaque sheet of paper and placed it on the overhead projector. We could see a spot of light (pinhole image) on the screen. She then sandwiched the paper-with-pinhole between two sheets of transparent polarizing film. Rotating the top sheet caused the spot of light to alternately dim and brighten, which is what we physics teachers would expect. But then she removed the top sheet and placed a transparent crystal on top of the pinhole. Surprise! We now saw two spots of light on the screen, instead of one! How come? It turned out that the crystal was Calcite, which is birefringent, meaning it has two different indexes of refraction depending on the direction of polarization of the light passing through it. So the polarized light from the pinhole was refracted at two angles to form two distinct pinhole images! Then, as Marilynn rotated the crystal (about its vertical axis), the spots rotated about each other while one would brighten and the other would dim, alternately "winking" on and off. Beautiful!

Next, when she sandwiched a transparent plastic protractor between the two sheets of polarizing film, we saw the image of the protractor on the screen, but it had pretty "rainbows" of color throughout. This indicated internal mechanical stress frozen into the protractor, resulting in index-of-refraction variations to produce the colors. Ann Brandon said that such protractors had a maximum stress located near the 45° mark, due to the process of manufacturing, and tended to break there. Marilynn then put a transparent, U-shaped plastic piece between the films, and as she squeezed the ends of the U, she produced internal stresses which resulted in beautiful rainbow colors within the projected image of the U. Next, she produced a colorful display by placing a transparent sheet between the polarizing films. The sheet was covered with criss-crossed pieces of cellophane sticky tape. Can you explain this?

She put two quarter wave plates on top of one another, to make a half-wave plate, and them between the polarizers. When she rotated the plates, there were 4 cycles in a full rotation, but only 2 cycles when she rotated one of the polarizing sheets. The half wave plate at angle q to the beam polarization rotates that polarization by angle 2 q.

She passed around the reference book Polarization of Light - Basic Theory and Experiment by Hollis N. Todd,  [Bausch & Lomb Inc. 1970] pp 24-25, which described quarter wave, half wave, and full wave plates.

01 May 2001 Arlyn Van Ek (Illiana Christian  HS, Physics)
brought in a magic liquid that appears to have the capability to restructure broken glass.  He took a Pyrex™ glass test tube, crushed it on the table, and  put the pieces into a beaker full of the magic liquid.  He then reached into the beaker and pulled out the unbroken test tube.  The magical properties of the liquid [Wesson Oil™ or Vegetable Oil, actually] are associated with the fact that its index of refraction is the same as glass, so that we could  not see the other (unbroken) test tube already inside the beaker, or the broken glass pieces remaining in the bottom of the beaker! Very sly, Arlyn!

05 December 2000 Bill Shanks (Joliet Central HS, retired; Joliet Jr College student)
titled his presentation: LED by the Light. So Bill lit up some LEDs (Light Emitting Diodes) for us. Pretty! He showed us how to do it for low cost. The LEDs were bought at Radio Shack (20 in a package for \$2), and a battery holder (American Science Center for \$ 0.50) and loaded it with 2 AA cells, which had been used in another application and were weak. The LEDs are rated at 2 volts and 20 mA, though they will light up at about 7 mA, he told us. Holding one up, he showed us that the long lead is the anode, the short lead is the cathode. (The big TV was connected to a miniature videocam to display a large image of small objects, and Bill frequently used this so that all of us could see clearly the small details on these devices. The videocam is available at the website http://www.allelectronics.com/) If connected in reverse, an LED won't light up. It is a diode, and will pass current in one direction, but not the other; it rectifies. Bill held up an AC-to-DC adapter that puts out various voltages: 1.5, 3.0, ...up to 12 v, bought at the American Science Center website http://sciplus.com/. Then he showed us an LED with 3 leads; the long lead is the cathode and the two short leads are anodes. When he connected the cathode to the negative end of the adapter and one of the anodes to the positive end, the LED glowed red. But when he connected the other anode to the positive end, it glowed green! If both were connected at the same time, the color was yellow! Appropriate to the Christmas season. Thanks for the good info, Bill, you sure brightened up our day!

06 November 2001: Bill Shanks (Joliet Central HS, Physics, Retired) Home Made Conducting Baton
Bill
made a conducting baton out of a dowel rod and handle, hooking two different light-emitting diodes [LEDs] in parallel, using current-limiting resistors and a step-down transformer to 6.5 Volts.  The circuit looked like this:

When the baton was held still, the LEDs at its tip glowed yellow, but then Bill swept it across the darkened room.  Thanks to persistence of vision, we saw alternating  green and red streaks of light produced by the diodes as first one and then the other lighted for about 1/120 sec each time.. Betty Roombos took a picture with her digital camera, and three sets of streaks were visible. Each streak was separated by small "dark regions" when the electrical potential was below the "LED flash point" of about 1 Volt.

Bill started to explain that each of the two 47 W resistors took the current passing through a different LED, but he was unable to complete this remark because his tongue got stuck inside  his cheek. Better luck next time at pulling the wool over our eyes, Bill!

04 December 2001: Bill Shanks  (happily retired from Joliet Central HS, Physics) Mixing of Colors
Bill
is taking a course using software packages such as Adobe Photoshop® or Photo Deluxe® to edit digital photographic images.  He showed us some very impressive images, including a shot of the Sawtooth Mountains in Idaho.  The slide had been damaged by creases or scratches, but by digitizing the slide image using a scanner and repairing the damage using software, he produced a beautiful 8 ´ 10 print

Bill remarked that the "layering" of images in these photo editors is truly amazing, and raised the question of whether image overlays are treated "additively" or "subtractively".  He tested the question by forming yellow, cyan, and magenta circles in the image editor with a (grey background), and overlaying each pair of circles:  He held up a page showing us these results in for overlapping regions:

 Subtractive Combination Overlap colors Color produced yellow + cyan green cyan + magenta blue yellow + magenta red

Bill led us to the conclusion that the colors are treated subtractively in the imaging program, as in the image of a color printer.  The inks in a desk-jet printer are yellow (red and green transmitted), cyan (blue and green transmitted), magenta (red and blue), and black (nothing transmitted).  Thus, when white light (all colors) is incident on a printed page, we obtain the above reflected colors, the rest being absorbed by the ink.

By contrast, the electron guns in a color television picture tube strike phosphors in the screen that emit light of specific colors:  red, green, and blue.  The colors are produced additively, with following results:

 Additive  Combinations Overlap colors Color produced red + green yellow red + blue magenta green + blue cyan

Porter Johnson mentioned that the "red" phosphors in picture tubes involve rare earth salts, and add significantly to the cost of the tubes.  Very interesting, Bill!

11 December 2001: Bill Shanks (retired from teaching; still learning about physics) How Bright is Bright?
Bill
remarked that luminous intensity is no longer expressed in terms of "foot candles" [brightness of a standard candle at a distance of 1 foot, but what is a standard candle, and whose  foot should we use?], and instead is expressed in SI units as follows:

 Unit / Source Definition Candela http://www.natmus.dk/cons/tp/lightcd/light_cd.htm The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. Lumen http://www.natmus.dk/cons/tp/lightcd/lumen.htm The lumen, the unit of luminous flux, is the luminous flux dF of a source of luminous intensity I (candella: Cd) in an element of solid angle dW is given by dF = I dW Lux http://www.natmus.dk/cons/tp/lightcd/lumen.htm The energy density striking the object is given in lumens per square meter, generally known as lux.

Bill noted that his LED sources were rated in mCd [milliCandella, presumably].  He showed us the following light sources:

 Source Description Light Source Output Slide Viewer Light Source 38 Watt Full Spectrum Fluorescent Bulb Like 150 Watt incandescent bulb Bluish Light 75 Watt Filament Like 500 Watt incandescent bulb Spotlight (Re-chargeable batteries) 1,500,000 Candela

Finally, he mentioned the following types of devices to measure luminosity:

• Bunsen Oil Spot Photometer [visual match for intensity].
• Light and Moisture Meter [obtained from K-Mart]--- light meter part is a photo cell
• CdS Photo Resistor, hooked to TVM [Transistor Voltmeter]. The resistance is a function of the incident light.
• Photo-transistor (current responds to IR and/or visible light).
• Silicon Photocell (produces a photocurrent to be measured and calibrated).

Very interesting, Bill

20 January 2002: Fred Schaal (Lane Tech HS, Mathematics) Screaming Orange Hat
Fred showed his stylish new hat, produced out of Ten Mile Cloth / Dynamic Textiles, Inc. Phone 718 631 5005 Fax 718 279 4104. He raised the question of the meaning of the label, which used the terms "dominant wavelength between 595 and 605 nm", "excitation purity not less than 85%", and "luminous factor not less than 40%".  The meaning is given in terms of the International Hunter Education Association [IHEA] specifications for "Hunter Orange" garments:

"The IHEA recommends the description of Hunter Orange as "having a dominant wavelength between 595 and 605 nanometers, a luminance factor of not less than 40% and an excitation purity of not less than 85%". Highland guarantees that Ten Mile Cloth, Camo Ten, Easy Ten, and TenAcious meets these specifications."

20 January 2002: Bill Shanks (Joliet Central, retired ) Fluorescence of Fred's Hat
Bill used a very bright flashlight to test whether Fred's hat was appropriately fluorescent. The standard test for fluorescence of a fabric is to shine an ultra-violet light on the fabric, and not whether the fabric "glows".  Unfortunately, the flashlight produced too much visible light, so that we could not see the fluorescent glow.  When the room was dark, there was no phosphorescent glow, either.

05 February 2002: Marva Anyanwu (Green School) [Handout:  Can Light be made by Breaking Sugar Crystals?]
Marva put Wint-o-green Lifesavers™ that contain sugar into a clear plastic bag that she tied shut, darkened the room, and after a minute or so she used pliers to break the lifesavers into pieces.  We looked carefully at the bag while pieces were being crushed, and saw flashes of light.  Why is the light being produced?

Electricity is used to make light in light bulbs, and fireflies make light through chemical processes.  In this case, the light is being produced as a result of mechanical stress [triboluminescence].  The lifesavers contain crystals of sucrose (table sugar), which are broken,  thereby releasing energy.  This energy excites atoms, which then emit a soft blue-white light.  If you prefer, you may simply chew the lifesavers in front of a mirror in a dark room.  The effect is described on the MadSciNetwork of Edible/Inedible Experiment Archive website: http://www.madsci.org/experiments/.  That site also contains the following information:

"Triboluminescence is the mechanical generation of light. Certain chemical bonds will generate light energy when the molecules are torn apart by mechanical crushing. Wintergreen Lifesaver candies contain some of these bonds. No other flavor of lifesaver candy (such as peppermint) will work in this experiment.  You are generating light energy by triboluminescence because each time you chew the candy your teeth are tearing apart the chemical bonds that were formed when the liquid candy was molded into a solid lifesaver. Wintergreen contains molecules that exhibit triboluminescence."

Marva also discussed an article on Stomach Bypass Surgery:  It's a serious step, but they saw gastric bypass as their best hope [20 Jan 2002; Kevin Davis Special to the Chicago Tribune: http://www.chicagotribune.com/].  She explained the procedures for treatment of morbidly obese patients, using a plastic model of the human stomach.  We had a lively discussion.

Good work, Marva!

We had an excellent session, top to bottom, this afternoon!

05 March 2002: Larry Alofs (Kenwood HS Physics) -- Glass Blocks for Optics
Larry
passed out solid glass blocks [12 x 12 x 1 cm], and showed us how to do several optics experiments with them.  First, we put an x-mark on a horizontal sheet of paper, stood the block up vertically on end on the paper over the x, and looked down through the top of the glass.  We could see the x through the glass; in fact several images of the x were visible, because of internal reflections of the glass.

When we looked into the glass from the side, we could not see the mark. Why?

Larry passed out laser pointers, as well as 5 cm pieces of solid glass rod about 1 cm in diameter. Then we put the  broad side of the glass block on a sheet of paper, traced around it, and shined the laser obliquely through a small face, using the section of glass rod to spread the beam.  Then, we traced the path of the incident beam and the beam transmitted through the block onto the paper, moved the block, and then sketched in the linear path of the beam inside the block, as shown.

We used a protractor to measure the incident angle (i = 48°) and refraction angle in the glass (r = 30°), and computed the index of refraction of the glass, using Snell's Law:  n = [sin i ] / [sin r ] = [sin 48°] / [sin 30°] = 0.74 / 0.50 = 1.48.  The accepted value is about 1.55.

Larry then showed a clear plastic spiral light pipe about 1 cm in cross-section diameter and about a meter in total length, and shined a laser pointer into one end.  We could see a little reflected light coming out of the sides of the light pipe, but most of the light emerged from the other end.  This is a super-duper model of a fiber optic cable.  Larry said that the glass blocks were expensive [\$10 each], and that the laser pointers were available from Harbor Freight Tools:  [http://www.harborfreight.com], but at the low price of about \$10 each..  We see the point, Larry!

19 March 2002: Larry Alofs (Kenwood HS Physics) -- Glass Blocks for Optics
Larry
gave some background information on where he developed his presentation on optics for the last SMILE meeting.  It was based on presentations in the series Active Physics [ISBN 1-891629-00-X], with 6 thematic units, which was developed by Dr Arthur Eisenkraft and others under the auspices of the AAPT and APS, and issued by the publisher It's About Time, Inc  [2002].

07 May 2002: John Bozovsky (Bowen HS, Physics) Laser Light Show
John
put on a Laser Light Show using a laser pointer [a HeNe Laser] and mirrors, which were bonded obliquely to shafts that are rotated independently at continuously adjustable rates by motors.  The laser beam strikes the surface of the first rotating mirror, and then it is reflected  onto the second rotating mirror -- and then perhaps to a third rotating mirror -- before being reflected into the room.  Because of the independent rotations of the reflecting mirrors, the laser beam traces an interesting pattern on the wall or ceiling, which is reminiscent of Lissajous Figures on oscilloscopes [http://www.jmargolin.com/mtest/LJfigs.htm] or mechanically, as done originally by Jules Antoine Lissajous [ http://www-groups.dcs.st-and.ac.uk/history/Mathematicians/Lissajous.html]. By varying the frequencies of rotation, we can alter the pattern of the beam, and set up closed figures in certain cases.  At last, we are stirred by the light!  Thank you, John.

24 September 2002: Fred Farnell (Lane Tech, Physics)  IMAX
Fred
described his visit to the IMAX Theater at Navy Pier.  The layout is different from that at the more familiar OMNIMAX theaters.  In IMAX Theaters, you wear a headset connected to 3-d glasses, and get a three dimensional coordinated visual and sonic image with reduced probability of getting headaches, as with the old stereoptic systems. And, just what is stereoptic vision?

stereoptic vision The perception of depth and three dimensions accompanying binocular vision resulting from differences in parallax producing different images on the retina of each eye.  See http://en.wikipedia.org/wiki/Stereopsis.
We see what you mean, Fred!

24 September 2002: Betty Roombos passed around her Viewmaster [http://www.vmresource.com/] loaded with 3-d photographs she had taken at a site of geothermal activity in Yellowstone National Park.  She said that such stereographic slides are easy to make; you just take a slide while leaning to the right, and then take one leaning to the left!  Many stereoptic cards and stereographic images are well over a century old!. Interesting, Betty --- and be careful when leaning over those geothermal ponds.

07 November 2002: Bill Shanks [Joliet Central, retired]    Mixing Colors
Bill
presented his attempts to use Christmas tree lights, which are once again available in stores, to demonstrate additivity of colors. He was motivated by an out-of-focus photograph that he took of a Christmas tree many years ago, in which the image of each bulb blurred into a colored "circle of confusion".  The idea was to use a lens to produce overlapping circles of confusion from bulbs of different colors, in order to see color mixing in regions of common illumination.  He described the general idea in the SMILE meeting of 12 December 2001. The idea is to discover whether this technique will indeed lead to these additive color relations:

 Additive  Combinations Overlap colors Color produced red + green yellow red + blue magenta green + blue cyan
To our delight, Bill did show us that red and green circles of confusion produced a yellow color in their region of overlap. (In practice, the blue light was too dim in comparison to the other lights, so that we were not able to see cyan and magenta very well.)  A wonderful and original idea, Bill!

07 November 2002: Arlyn Van Ek [Illiana Christian HS, Physics]      Identifying Colors in a Dark Room
Arlyn
described an experiment that he had seen at a convention, in which a sheet of paper of unknown color was illuminated in a dark room by lights of various colors.  The goal was to determine the "true" color of the paper from its appearance with various colors of illumination.  As a sequel he mentioned that, when the American flag is viewed in intense blue light in a dark room, the red stripes appear to be black.  For an interesting discussion of the Physics of Color, see an article in the July 2002 issue of Physics Today.  See also the Java Applet on the Color Matching Game, http://www.cs.rit.edu/~ncs/color/a_game.html, on the Introduction to Color website: http://www.cs.rit.edu/~ncs/color/. Larry Alofs mentioned that an ultraviolet LED is available at All Electronics:

Cat # ULED-1  [5mm Ultra-violet LED. Emits blue 395nm UV light. Water-clear lens. 3.7 Vdc, 20 mA. 15 degree beam pattern. Ideal for counterfeit bill detection, detection of fluorescence in minerals, black-light light poster lighting]:  http://www.allelectronics.com/.

Very interesting, Arlyn and Larry.

07 November 2002: Hoi Hyunh [Clemente HS]     Seeing Infinity with a Magnifying Glass
Hoi
placed an object a distance d from a magnifying glass, and observed a [real or virtual] image at a distance x from the object, as indicated below:

```
```
The magnification of the image is given by the formula M = - d / (x -d) . Note that, when x = d, the magnification becomes infinite.  Note that the object distance d and the image distance d are given by the lens maker's formula, 1/d + 1/(x-d) = 1/f, where f is the focal length of the lens.  This formula may be converted from this Gaussian form to the Newtonian form, ( d - f) ´  ( x - d - f) = f2.  Very interesting, Hoi!

03 December 2002: Larry Alofs  [Kenwood Academy, Physics]      Diffraction versus Refraction
Larry
produced a diffraction grating with 530 grating lines per millimeter, corresponding to a spacing d = 1.89 microns between lines.  He passed light from a standard red diode laser through this slit and onto the white board, showing 3 spots, which meet the grating condition d sin q = n l, the central spot corresponding to n = 0 and the two side spots for n = ±1. Larry pointed out that sin q = x / L = l, where x is the separation distance between spots, L is the distance from the grating to the board, and l is the wavelength of light.

As an extension of the exercise, Larry asked what would change if we replaced the red laser light source with a green one.  We decided that the wavelength l of light hitting the grating would be decreased, so that the distances between spots would also decrease.  Larry took out a green light solid state laser which has a 3 Volt power supply. He had ordered the light from a Harbor Freight Catalog [http://www.harborfreight.com/] for about \$200, but it did not appear in their most recent catalog.  He put the green laser on the same stand as the red one, so that both could pass through the slit.  We observed that the green side dots were about 20% closer to the center dot than for red. Very impressive, Larry!

To distinguish the color separation produced by the grating [interference] from that obtained with a glass prism [refraction], he suspended a fairly large prism in front of the two laser light sources.  We could see that, indeed, green light was refracted more than red light, because the index of refraction for glass and many other materials is greater for green than red, but that the images shifted by about 1% of the amount observed previously with the diffraction grating. Beautiful!

Now we see the light, Larry!

03 December 2002: Don Kanner [Lane Tech HS, Physics]    Camera Obscura
Don
began by describing a recent television program that explained the role of the Camera Obscura [http://www.rleggat.com/photohistory/history/cameraob.htm] in the late renaissance, in which the goal was to produce faithful images of portraits, rooms, and even landscape scenes.  The program highlighted the conclusions presented by David Hockney, Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters [Viking Press 2001: ISBN 9-6700-30260].  Hockney has suggested that paintings such as da Vinci, Caravaggio, Velázquez, and van Eyck were actually created using optics and lenses; see http://www.acmi.net.au/AIC/CAMERA_OBSCURA.html..  The Flemish artist Jan Van Eyck [born in 1434] may have used a lens [http://www.ibiblio.org/wm/paint/auth/eyck/] to produce the image of the little dog at the bottom of the painting Arnolfini Wedding Portrait.  Indeed, in a mirror behind the couple, we can see the wedding party, as well as a person covered with "black-out cloth" and looking through a hole.  Hockney does not consider his work an exposé of the great masters;  instead he feels that artists have always made use of the technology available to them in creating their images, and the Camera Obscura [or dark room!] may have been more widely used than previously thought.  The achievements of these artists stand as one of the greatest monuments to artistic genius of all time.  In the modern age, the dominant mode of art consists of digital images, which are more flexible and adaptable than the traditional easel, canvas, brushes, and paint.

One could produce images suitable for tracing onto paper using (1) a small hole to form a pinhole camera, (2) a lens to focus light, or (3) a spherical mirror to focus light.  Since the first method produces very dim images, even inside a darkened room, and since the technology to produce high-quality large images has been available only for about two centuries, Fred thought that curved mirrors would work best.  He brought a large spherical mirror about 50 cm in diameter, with a radius of curvature of about R ~ 2 meters, corresponding to a focal length of  f = R/2 ~ 1 meter.  On a screen we could see the inverted, blurred  image of a person standing in front of the white board, by reflection off the spherical mirror.

Porter mentioned the film Artemisia [1997; French language], which deals with the life of Artemisia Gentileschi (1593-1653), [http://www.u.arizona.edu/ic/mcbride/ws200/gentil.htm] one of the first well-known female painters.  In that film the artists view landscape scenes through a small hole, placed about a meter behind a 6 ´ 6 lattice network that lies in a vertical plane, to set the correct perspective while laying out an image.   Don mentioned that  Albrecht Dürer of Nürnberg also made sketches using a gadget with strings and a grid.  At last we see the light!  Thought-provoking and interesting, Don!

10 December 2002: Bill Shanks [Joliet Central HS, Retired]    Addition of Colors using Spotlights and Colored Cellophane
Bill
brought in 3 rechargeable spotlights with rated luminosities of around 2 million candle power, and covered them with red, green, and  blue colored flexible cellophane sheets -- found in some stores as gift-wrapping -- which he held in place around the edges of the spotlights with sewing hoops.  He was then able to produce high intensity beams of red, green, and  blue  light.  Bill was wearing his Choral Costume -- since he was on his way to an evening concert-- complete with bright red bow tie. The red bow tie shined resplendent in the red light, but looked quite dark in both green and blue lights.  Fish that habituate the 10-100 meter depths of ocean are often colored red in order to be almost invisible in the ocean, which at those depths receives light predominantly in the violet part of the spectrum.  We expected, and mostly got, these combinations:

 Color Combination Resultant Hue red + green yellow red +  blue magenta green +  blue cyan red + green +  blue white

The patterns of colors on the white boards produced by the (umbra and penumbra) shadows of Bill's hands were very fascinating! Actually, we found that that the red and green cellophane paper were fairly pure, but that the allegedly blue cellophane paper behaved more like blue-green in our studies.

We have seen the lights! A colorful and interesting  visual presentation, Bill!

25 February 2003: Bill Shanks [Joliet Central, Retired]      Knock Your Eyes Out
Bill
began by putting five colored sheets of construction paper onto the white board in the classroom:

```
```
First he shined a powerful spotlight on the red sheet, and we looked steadily at that sheet. After 30 seconds or so, we began to see a greenish halo around the sheet, and then it seemed as though the color in the central region of the red sheet became less intense. We shifted our gaze to somewhere else on the white board, and began to see cyan, the color complementary to red.  We repeated this procedure for the other sheets, and saw the following after-image colors:
 Color Red Green Yellow Orange Blue After-image Cyan Magenta Deep blue Blue Yellow-orange
Why is this happening to us?  In the original theory of color vision developed by Hermann Helmholtz, there were 3 color receptors in the eye, corresponding to the primary colors (red, green, blue).  When you look at, say, an intense red field of view, the red color receptors become fatigued --- unable to restore the chemicals necessary to "see red" in the retinal area.  As a consequence, the interior red field becomes less intense.  When we look at the white board, our red receptors are temporarily out of commission, so we see the complementary color cyan, a mixture of blue and green.  According to the modern theories of color perception, these three types of color receptors accept a range of colors (wavelengths), but are most sensitive at (red, green, blue), respectively.  For details see Light Science:  Physics and the Visual Arts by Thomas D Rossing and Christopher J Chiaverina [Springer 1999] ISBN 0-387-98829-0.

We are dazzled by your brilliance and great ideas, Bill!

25 March 2003: Katherine Hocker [Bloom Trail HS, Physics]     Home-made Spectroscope
Katherine
expressed frustration with traditional spectroscopes, in that students took almost a full lab period to be able to see simple diffraction images, etc.  She passed out several of her home-made spectroscopes, with a CD serving both as a diffraction grating, and as the base of a closed cylinder constructed from card stock.  A 3 cm x 0.5 cm slit is cut at the edge of the horizontal, circular top end of the cylinder, the short dimension being tangential and the long dimension radial.  A similar slit is cut at the bottom of the lateral cylindrical surface, just under the first slit and oriented with the long side up.  When we looked through the bottom slit while just under a bright ceiling light, we could see the  diffracted image, which contained the full visible spectrum For more details see the website The Compact Disc as Diffraction Gratinghttp://230nsc1.phy-astr.gsu.edu/hbase/phyopt/grating.htmlWow!

Pretty stuff, Katherine!

25 March 2003: Chris Clausing [Bloom Trail HS]      CD Spectroscope [handout]
Chris
made a CD spectroscope using the same procedure as that described by Kathy Hocker in the Math-Physics section today.  We looked at various light sources in the room using the spectroscopes that she had made.  Here is some information from her handout sheet:

"A spectroscope is a device that separates light into its component colors.  The way a spectroscope does this is to make use of something called a diffraction grating.  Light reflects and refracts through this diffraction grating, and the different colors of the spectrum all refract a little differently.  This is how the colors are separated into the colors of the rainbow.  A CD contains a large amount of information encoded onto its surface.  This information is stored in concentric rings so that it can be read by a laser beam while the disk is spinning.  These concentric rings can act as a diffraction grating if the light hits them just right. Around the room are 6 different light sources.  Each of the light sources corresponds to a different element.  Your task will be to identify the elements, based on their line-emissions."

Each light source would be expected to have a different emission pattern, since that is dependent on the nature or the source, the energy levels for electronic transitions, etc.

Interesting stuff, Chris!

06 May 2003: Bill Shanks [Joliet Central HS, retired]        Persistence of Vision + Other things
Bill
showed us his latest toy, a Skyliner Electronic Message Maker, which he got recently at WalMart.  When the device is turned on, it plays a tune, and two sets of blinking LED's are lit.  When it is twirled in a circle in the vertical plane, the persistent images of words are created.  Fifteen phrases, such as SMILE and BE HAPPY are programmed into the device  -- each with a different distinctive and possibly recognizable tune!  Or, you can program you own messages.  To operate it, spin it around in the air --- the words magically appear before your eyes!

Bill briefly discussed the phenomenon of persistence of vision, which explains how this device works -- as well as the image on a television screen.  The visual cortex lays out an image in the brain, which persists during loading, so that we can get the big picture, or at least the big perception.  Bill quoted a former physics teacher, who recommended saying  "So it seems to be now the thing I think I see", rather than "Now I see".

Bill then passed around LED Pens of various colors, which he obtained at Walgreens --- which also can be ordered at http://www.wholesaleforeveryone.com/Merchant2/merchant.mv?Screen=CTGY&Category_Code=LED. He showed us three LED Pens --- red, yellow-green, and blue, which he used as light sources.  He turned on various combinations in the darkened room, and showed us the color circles they projected on the white board in front of the room.

Bill then showed his next toy, a search light with two white light sources, one green LED, and a xenon bulb., which he had obtained from Berland's House of Tools: http://www.berlandtools.com.    This device is similar to the Trident Headlamp:  http://www.nextag.com/streamlight-trident-headlamp/search-htmlBill intended to use this device as a skunk repellent, and did so with limited success. The eyes of many forest animals reflect light back to the source, so that the eyes appear to glow in the dark.  For details see the websites Nocturnal eyes: How do animals see in the dark?http://ebiomedia.com/gall/eyes/nocturnal.html or Why do cats eyes glow in the dark?http://dspace.dial.pipex.com/agarman/bco/fact4.htm.  [Unfortunately, this technique might work only on the four-legged variety of skunk!].  Bill used the white light to demonstrate retinal fatigue, by shining it on a magenta sheet of paper on the white board in the darkened room.  After a minute or so, we looked away from the sheet, and we saw a green image in the shape of the paper sheet.  Fascinating!

We think we see the light now!  Thanks, Bill!

09 September 2003: John Bozovsky [Chicago Discovery Academy at Bowen HS, physics]        Now You See It; Now You Don't
John
began by recounting the sad tale of the mathematics teacher who was detained while attempting to board an airplane at Heathrow Airport, London, UK.  He had a graphing calculator, compass, and protractor among his effects. He was suspected of belonging to the Al-gebra group, and was accused of attempting to board a plane with instruments of math instruction. Verrry interesting, John!

John held up a large and colorful poster with a high-quality image of patterns.  By staring at the poster and allowing your eyes to focus at a distant object, most people may be able to see three-dimensional images.  John had many different posters, which he held up for us to see. He offered to sell them to us at \$2.00 each.  He had obtained them at a warehouse. Some of us could see dragons and dinosaurs chasing and being chased across the landscape, whereas others were unable to see the three dimensional images.

09 September 2003: Larry Alofs [Kenwood Academy, physics]        Crude Explanation of the Effect
Larry
explained that stereographic images come from nearby repeated patterns present in ordinary images, that lead us to visualize a three-dimensional super-image. Consider the following image:

```

A     B      A B
|           |
D   A   B   C   D   A  B   C   D   A

|                   \
(left)                 (right)
eye                     eye
```
You will interpret the objects A and B in perspective, with A closer than B. Thus, three dimensional images appear, when one focuses on both A's, making them appear as a single image further away. Correspondingly, the image of A appears closer to us. For additional information see the Citeseer website http://citeseer.ist.psu.edu/context/660573/0, from which the following has been excerpted:
"...a stereoscope. It might seem that stereopsis necessarily requires two separate pictures, or at least some method of splitting a single picture into two to give each eye a separate view (using, for instance, red green filters, polarized light, or interference, as in holograms) Recently, however, Tyler and Clarke (1990) realized that a pair of random dot stereograms can be combined together, the result being called a single image random dot stereogram (SIRDS) or, more generally, an autostereogram. Essentially, one overlays the two separate random dot patterns carefully placing the dots so that each one serves ....
....out that very convincing images with vivid depth can be constructed in this way, and the advantage of this ingenious approach is that no special viewing equipment is required. It does take a little practice to see depth in the pictures, but the experience is very satisfying when first achieved. Tyler and Clarke (1990) described a simple but asymmetric algorithm, which meant, for example, that some people can only see the intended effect when the picture is held upside down. This paper presents a new, simple, and symmetric algorithm for generating single image stereograms from any solid model. It also avoids ...."

We think we get the picture now!  Thanks, John and Larry!

09 September 2003: Ann Brandon [Joliet West HS, physics]      Space Inversion Mask
Ann obtained a white face mask from Oriental Trading Company: http://www.orientaltrading.com/: [PLASTIC WHITE FULL MASKS, Item Number: IN-25/1185]  We viewed the back (inside) of the mask as it was being illuminated from below by the lamp from an overhead projector.  Curiously enough, it appeared to us that we were seeing the front of the mask.  How come?  Ann explained that we use shadows to establish our orientation of an object, and that we are accustomed to seeing objects as illuminated from above.  In this case, the shadows appear the same as when the front of the mask is illuminated from above.  Thus, we think that we are seeing the front of the mask.  Simple but profound!  Very good, Ann!

23 September 2003: Jane Shields [Calumet Academy,  science]        Why is the Sky Blue?
Jane
took a glass of water and added a small piece of hand soap to it, and stirred the water.  As the soap dissolved, the water turned cloudy (milky). Jane then lit a candle, and we darkened the room.  It was quite evident that, when we looked at the candle through the water, the candle itself appeared to be reddish (yellow-orange, actually), whereas when we held the candle in front of the glass the water behind it seemed to be blue. This is one manifestation of Tyndall Effect, which explains that small particles suspended in water scatter blue light more efficiently than blue light.  Lord Rayleigh showed that  particles much smaller than the wavelength of light [0.5 microns, say] scatter light according to the inverse fourth power of their wavelength, so that blue light is scattered around 10 times as much as red light.

Bill Shanks used his new LED light sources [described below] to show that blue light is indeed scattered significantly more than either red or green light.

But now, is the sky blue because of dust in the atmosphere, or is that explanation incomplete?  For additional Information see the article Why is the Sky Blue? [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html], on the website of  USENET  Frequently Asked Questions in Physics, http://math.ucr.edu/home/baez/physics/, from which the following has been excerpted:

"A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.  When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight." ...

"The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859.  He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red.  This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in.  From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank".  ...

"Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere.  Even today, people sometimes incorrectly say that this is the case.  Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering." ...

We see the light now!  Thanks, Jane!

27 January 2004: Bill Shanks [retired physics teacher, Joliet Central]        "LED"ing You On
Continuing in his unofficial role as MR LED MAN, Bill showed us his latest LED acquisition. First he reminded us of his older system, with separate red, green, and blue LED lights.  Then he showed us the new light-fiber pen, obtained at no great personal expense from Walgreen's, in which light from LED sources is partially reflected in the filamentary fibers fanning out from the end of the pen.  Thus, you can see the colors through the sides as well as at the ends of the fibers.  When he turned out the lights and waved the pen in the air, we could see remarkable images, illustrating persistence of vision, color mixing, and the like.  [For information on a Light-Up Fiber-Wand, see the Identity-Links website http://www.identity-links.com/light-up/light-up-fiber-wand.html] It was a very fascinating visual display, and we thank you greatly for showing it to us.  Nice work, Bill!

23 March 2004: Bill Blunk [Joliet Central, Physics]           Plane Mirrors
Bill
attached a mirror to the blackboard with magnets, and stood in front of it.  He noted that, in the mirror, "left" and "right" appear to be interchanged.  In particular, when he held up his right hand, his image lifted its left hand, etc.  To clarify the situation, Bill recruited Ann Brandon to play the role of his mirror image.  Bill (facing North )and Ann (facing South) were looking directly at each other, while the imaginary mirror between them was vertical and in the East-West plane.  When Bill lifted his right hand, Ann lifted her left hand, as expected for a mirror imageBill then pointed out that when he moved his Eastern hand, Ann also moved her Eastern hand.  However, when Bill pointed his finger North (toward Ann), Ann pointed her finger South (toward Bill)Bill then said, "Isn't a mirror image just a front-to-back reversal?"  In other words, both the object and image agree on the directions parallel to the mirror (East or West, up or down), but disagree as to the direction perpendicular to the mirror (North - South).

As a further test of these ideas, Bill showed us front-to-back reversal by mirror writing.  For more information see the website Mirror Writing:http://en.wikipedia.org/wiki/Mirror_writing.  The following information on Leonardo Da Vinci is excerpted from that page:

" ...He is also the most celebrated mirror writer to date. Most of his manuscripts, letters and meticulously illustrated notebooks were written in mirror image. No one knows why he wrote this way but two theories suggest convenience and security. ..."
Next Bill asked the following question:  We often "back away" from a mirror to get a more complete view of ourselves.  Does this work?  To find the answer, Bill stood in front of the mirror on the blackboard about 1.5 meters away, a held a meter stick vertically alongside his head.  He noted the positions of the top and bottom edges of the mirror, as read on the reflected image of the meter stick. Then, he backed to a distance of about 3 meters from the mirror.  Guess what?  He could see the same amount of the meter stick in the mirror.  How come?

Bill then drew the following sketch of rays passing into the observer's eye after being reflected by the mirror:

```      ===================
right side   eye     left side
X         E         X
\       / \       /
\     /   \     /
\   /     \   /
\ /       \ /
\=========/  mirror
\       /
\     /
\   /
\ /
I
image of eye
```
Houston, we have a problem! It seems from the diagram that we can see a width (of ourselves) that twice the width of the mirror, and only that much --- no matter how far we are located from the mirror. Now, just why do we instinctively "back up" from a mirror in order to see more? Bill suggested that, in fact, our early experiences may have been with the mirror on mother's dresser.  We would get a better view of ourselves by backing up, to avoid blockage by the dresser itself. Very interesting, and perhaps even true!

You took us through the Looking Glass so that we could see our own world more completely! Great job, Bill.

06 April 2004: Bill Blunk [Joliet Central, Physics]           Jacob's Ladder + Signaling with Plane Mirrors
Bill
began by showing us Jacob's Ladder, which is described on the Science First website in the article How to Build a Jacob's Ladder  http://ffden-2.phys.uaf.edu/212_spring2005.web.dir/kenneth_sweet/. A Neon Sign Transformer is used to set up a high voltage arc between a pair of vertical, nearly parallel wires.  The wires were about 30 cm long, and closer at the bottom end (about 5 mm), tapering gradually farther apart toward the top.  The arc begins at the bottom end of the wires, and it moves slowly and erratically up the wires, and then disappears at the ends --- as with the angels of Jacob's ladderHow come?  The arc is hot, ionized air, and -- being less dense than the surrounding air -- it is buoyed up, and rises.  Bill fanned the air around the arc as it began to move upward, and it reversed its course, because the ion path was pushed downward by fanning.  Very interesting! Bill put a piece of paper through the arc, and moved it quickly around for a second or two.  We saw that the paper had many small holes burned into it --- those are produced at the rate of 120 per second by the alternating current.  Fascinating!

Bill then reminded us of Colin Fletcher, whose travels through the Grand Canyon are described in the book The Man Who Walked Through Time:  http://www.amazon.com/Man-Who-Walked-Through-Time/dp/0679723064.   He took a mirror along to signal aircraft by reflected sunlight, and so notify them of his location, so that they could drop food to him.  However, he had great difficulty in hitting the aircraft with the reflected image of the sun, and was forced to build a large fire to identify his location.  How could he have reflected the sunlight to the aircraft?  Bill set up a central light source (a fiber optic illuminator -- a bright light), and passed around some signal mirrors, which are polished stainless steel plates with a hole at the center. The idea is to look through the hole in the mirror at the object to be illuminated (aircraft). While keeping the object in view, turn the mirror. Sunlight coming through the hole in the mirror appears as a spot of light. Turn the mirror so that you can see the spot on your face by its reflection from (your side of)  the mirror.  Adjust the mirror so that the bright spot falls on  the hole of the mirror and in the line of sight of  the airplane.  Then, the reflected sunlight will hit the aircraft, as indicated in the diagram below:

```                               (mirror)
|      P  (aircraft)
|     /
(bright spot on face)*    |    * (image of bright spot)
\   |   /
\  |  /
\ | /
\|/
hole -->
/|\
/ | \
/  |  \
(eye) O   |   \ (Sun)
(mirror)
```

Bill, this one really hit the spot!

06 April 2004: Ann Brandon  [Joliet West, Physics]           Forming Images with Plane Mirrors
As we watched Ann used chalk to draw a semicircle on the front desk -- concave facing us.  She used a large wooden protractor as a guide. She placed about a dozen, small mirrors (10 cm high and 3 cm wide) side-by-side around and tangent to the chalk semicircle.  The mirrors were held vertically -- 10 cm side up -- on wooden blocks, facing the center of the semicircle.  She then placed a lit candle at the center of curvature.  Ann picked out a person at the back of the room, and asked him to help her adjust each mirror, so that he could see the reflected image of the candle in each of the mirrors.  We then saw that the mirrors had been re-arranged at regular intervals along a parabolic arc, and the candle was at the focus of that arc! Great!  Thus, we can focus light with an array of flat mirrors, by adjusting the location of each image appropriately.  We each walked by the location of the image, to verify that it had been focused there.  We then placed a light source at the back of the room, and saw that reflected light bathed the candle itself.  Furthermore, there was a bright reflected image produced by each mirror, in which we could see the candle superposed on the light source. Neato!

Beautiful images, Ann!

04 May 2004: Karlene Joseph  [Lane Tech HS, Physics]           Birthday Candles and Spectroscopes
Karlene
recently purchased from her local drugstore a set of Color Flame Birthday Candles [http://jdjiaxuan.en.alibaba.com/product/50003665/50029791/Color_Flame_Birthday_Candle/Color_Flame_Birthday__Candle.html]: 5 ´ 50 mm, which produce red, green, yellow, blue, and purple flames  and last for about 8 minutes Karlene passed out a supply of CENCO Quantitative Analysis Spectroscopes -- CP-30105-00, available from Sargent-Welch at  http://sargentwelch.com/product.asp?pn=CP30105-00_EA.  She lit the red candle, and we each looked at the spectral lines obtained for it, which included reddish hues, as well as other colors. After a few minutes, she lit the green candle, let us look at it for a few minutes -- and then the yellow -- and then the blue --- and then the purple candle.  It was fascinating to see the light from each candle, decomposed into its component wavelengths, and to associate them with different chemical elements.

Even though it wasn't our birthday, we saw the light! Thanks for sharing this with us, Karlene!

26 October 2004:  Bill Colson [Morgan Park HS,  mathematics]           Interdisciplinary Projects with Camera Obscura and Pinhole Camera
Bill also called attention to the article Unified Vision by Urmila Subramanyan, which appeared in the October 2004 issue of Teacher Magazine --- to see the article you must register on the website http://www.educationweek.org/tm/toc/2004/10/01/. The article concerned Ralph Howell, photography teacher at St Mary's Hall school, who first transformed a carnival wagon into a camera obscura   [http://www.acmi.net.au/AIC/CAMERA_OBSCURA.html] and subsequently has created a number of interdisciplinary class projects based upon the pinhole camera [ http://www.exploratorium.edu/light_walk/camera_todo.html].  To Ralph Howell, anything's a camera.  Just poke a pin through it.  Pinhole cameras are anything but routine, according to Howell.

Very interesting, Bill!

23 November 2004: Earl Zwicker  recently saw the film Polar Express -- twice.  He saw it in an IMAX theatre, as well as on an ordinary screen.  This film was made by a process developed by Pixar Technology  [http://www.pixar.com/index.html], in which a large number of small mirrors are attached all over the actor's body.  Lasers rapidly scan the scene, and the reflected light is recorded to produce a map of the motion of the actor.  Using this map, cartoon characters are then given true-to-life motion.  For details see the website http://www.pixar.com/howwedoit/index.html#.  Thus, Tom Hanks could play four different characters.  The new image is created by A Thousand Points of Light! Fascinating, Earl!

14 December 2004:  Lee Slick [Morgan Park, physics]          Film Canister Optics (a give-away available in large quantities)
Lee  showed us how to use a film canister and push pin to show inverted optical images, along the lines of his presentation at the MP SMILE class of 10 December 2002, from which the following is excerpted:

"Image Inverter:  Lee passed out a cylindrical film canister, complete with plastic cap, along with a plastic push pin,  to each of us.  We used the push pin to poke one hole in the center of the cap, and then put three holes very close together at the center of the base, to form a triangle.  We then pushed the pin through the lateral surface of the canister, in order to grasp it.  When we looked at a bright region on the wall through the single hole in the cap, we saw the three hole triangle in the base.  However, when we rotated the canister about a vertical axis passing through the center perpendicular to the lateral surface, we observed that the triangle had become inverted.  How come? For additional details see The Human Eyehttp://en.wikipedia.org/wiki/Human_eye and The Quaker Oats Canister: http://www.wackyuses.com/experiments/quakeroatscamera.htm. Karlene Joseph remarked that she also used the canister lid as a pin hole camera.  When we moved the push pin across the field of view and on the other side of the hole in the lid, we saw it move in its direction of actual motion.  However, when we moved it in that direction while held between our eye and the hole in the lid, it appeared to move in the opposite direction.  Very interesting, Karlene ..."

Thanks, Lee.

07 December 2004: Walter Kondratko [Steinmetz HS, chemistry]      Spectrums and Colors (Handout)
Walter had several tubes filled with various gases (one gas per tube: hydrogen, helium, mercury, etc.).  The tubes can be charged with an electric current, the energy from which will excite the atoms.  When the atoms return to their ground state, they will release energy in the form of photons (colors).  The energies (colors) in each case are characteristic of each element, and can be distinguished using a small diffraction grating.  White light (the control lamp) gives a complete continuous visible spectrum:

Red - Orange -Yellow - Green - Blue - Indigo - Violet --- ROYGBIV
The hydrogen lamp gives a number of discrete bands, the “hydrogen spectrum.”  We then tried mercury: a different set of discrete spectral bands, the “mercury spectrum.” Then neon: a more complicated spectrum, with most bands in the orange region.

We then looked at the white light again, but through solutions of various chemicals (Potassium Permanganate-- KMn04, etc.)  The absorption of characteristic wavelengths of light by each chemical will cause the (previously) continuous spectrum to be missing certain colors.  Each solution resulted in a characteristic pattern of colors being absent from the spectrum.

Walter received this equipment on loan because he participated in the ChemVan program at Chicago State University.

This was a terrific activity, which everybody thoroughly enjoyed!

25 January 2005: Ben Stark [Professor of Biology, IIT]  Colors of Light
This is a mini-teach that Ben tried out last week with a fourth grade class for which he is the "science volunteer"; it seemed to go over quite well with them, and it can be adapted for the upper grades quite easily. It combines physical chemistry with physics.

He started out shining a bright flashlight through a prism and displaying the resultant rainbow on the wall.  We then discussed how reflection/transmission of some of the colors and absorption of others by various dyes/colored chemicals, leads to the property of "having a certain color." We then talked about how something that appears white does so because all the colors in the visible spectrum are reflected and none absorbed, and conversely how something which is black is so because it absorbs all the colors. This then led to the demonstration with the radiometer (also called a "light mill").

A great discussion about this apparatus can be found at the website How Does a Light Mill Work? by John Baez: http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html. Basically, the vanes of the windmill type device inside the light mill each have a dark (light absorbing) and light (light reflecting) side and the 8 sides of the four vanes are arranged: light - dark - light - dark - ... .  The light is absorbed to a greater extent on the dark side than on the light side of each vane, and this light is converted to thermal energy which heats the air near the dark sides.  The hotter air molecules have more kinetic energy than the cooler air molecules elsewhere in the radiometer, and thus give a relatively greater push to the dark side, than to the light side, of each vane.  Thus, light shined onto the vanes of the apparatus will push the vanes in a direction in which the dark sides seem to be repelled by the light.

Ken Schug then led us in a further discussion of this interesting phenomenon. We discussed how the quantum properties of electron states leads to the absorption of the various wavelengths of light (raising the electron to a higher energy state), how the reversal of the absorption of the electron returns the electron to a lower energy state releasing either light (fluorescence) or thermal energy (what happens with the radiometer), and the photoelectric effect.

02 May 2005:  Bill Shanks [Joliet Central, retired]              Ray-tracing on Blackboard Made Easy!
Bill
showed the Stanley 45-101 MaxStick Straight Edge, which he had obtained recently at Menard's.  It was marked in inches through 24", but could easily be modified by pasting on a 60 cm metric scale.  He attached flexible magnetic strips to the back, so that it could be used for blackboard optics, as well as other things.  He used the MaxStick for illustrating refraction, Snell's Law, mirages, total internal reflection, and a two-dimensional corner reflector.  Then he showed ray construction for a thin lens, laying out the symmetry axis, the plane of the lens, and the location of the principal foci.  Bill recommended the term "principal focus", rather than "focal point" to describe the point at which parallel light is focused by a  converging lens.  He illustrated that the images from a simple converging lens are always inverted.  In particular, the images on the retina of the eye are inverted, and the brain corrects for that.  He called attention to some experiments with Upside Down Glasses http://www.madsci.org/posts/archives/mar97/858984531.Ns.r.html, which indicated that it takes over a week for the users to see "normally" with these glasses.

Good teaching tips! Thanks, Bill.