High School Mathematics-Physics SMILE Meeting
1997-2006 Academic Years
Electromagnetic Waves

28 April 1998 Bill Shanks [Joliet Central High School; allegedly retired]
Bill described his quest to make an LED flashlight. The LED devices require 2.2-2.7 Volts to fire [for sufficient energy to overcome contact potentials and to produce photons], and thus will not work with 1.5 Volt flashlight batteries. However, with 3 Volt Lithium Cells [available cheap at Walgreens and, no doubt elsewhere] he was made a light. The LED itself has a short lead [-] and a long lead [+], and on the inside the [-] side is connected to a large electrode, whereas the [+] side has a small electrode inside. The Li battery must be used in connection with a "ballast" resistor, since 3 Volts will lead to "thermal runaway" and breakdown---although the latter point is mollified somewhat by internal resistance of the Li battery. [small batteries have fairly high internal resistance]. He also had a Blue LED that operates with a 6 Volt battery.

Comment by Porter Johnson: The Planck Formula

E = h n = h c / l
can be switched to practical units by using the relation
h c = 1238 electron Volts · nanometers .. or .. [eV·nm].

Thus, the wavelength l [nanometers] corresponds a battery voltage V [Volts] := 1238 / l[nm] :

Light Color Wavelength [nm] Voltage [Volts]
Red 700 1.8
Orange 600 2.1
Green 500 2.5
Violet 400 3.1

28 September 1999: Bill Shanks (Joliet Central HS, ret)
showed us some interesting physics using a Solar & Windup Radio (available at Sam's Club, $69); see http://store.sundancesolar.com/frraamsowira.html. An array of solar cells on top of the radio converts incident light (electromagnetic energy) into electrical energy to power the radio, which vibrates its speaker cone (mechanical energy) producing sound waves (acoustical wave energy). As the angle of incidence of the light from the source is changed (by tilting the radio, for instance), the amount of light on each unit of area of the array is decreased, decreasing the available energy, so that the sound output of the radio will decrease toward zero. Bill then made 55 turns of a crank on the radio, winding up a spring (mechanical potential energy). As the radio was played, we could see the crank slowly turn as the spring unwound and the mechanical energy was converted (with a built-in generator) into electrical energy to power the radio. Turning up the volume of sound caused the energy to be used more quickly, and the crank turned faster! What a pretty example of the interplay between different kinds of energy! Beautiful, Bill!

Bill also had three mirrors set up orthogonally as a "corner mirror," or corner reflector [http://www.exploratorium.edu/snacks/corner_reflector.html] which reflects any beam of light hitting them directly back at the source. How does it work? Can anyone explain?  For a discussion of corner reflectors left on the moon by astronauts, see the website http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html.

01 February 2000: Ann Brandon (Joliet West HS)
showed us a demo that she picked up at a national AAPT workshop. With a Radio Shack amplifier-speaker in one hand, and connected by wire to a small photo-resistor at its other end, Ann held the photo-resistor near fluorescent and incandescent light sources, and we heard very faint 120 Hz hum. Then she took a TV remote and aimed it the photo-resistor - producing a series of rapid "clicks" or "pops" when she activated the remote. The remote was sending its coded IR signal and it was detected by the photo-resistor which we then heard as sound generated by the amplifier-speaker. Great!

Finally, a flame (lighter) was held near the photo-resistor, and Ann ran a hair comb back and forth so its teeth interrupted the IR coming from the flame to the photo-resistor. This produced a sound like sawing a piece of wood, much to our amusement! Much fun, and good physics! Thanks, Ann!

19 March 2002: Bill Shanks (Happily Retired Physics Teacher) -- LED Exit Signs
Ever the watchful shopper, Bill found an LED exit lamp on a close-out sale at Home Depot.  The regular price of the lamp, which contains 22 LEDs in parallel with resistor and capacitor, and which runs off 120 Volts, is about $15.  The product was called Sure Lite Led Lite Styx Exit Retro Kit, product H410850.  A similar product can be obtained at website http://www.surelites-lighting.com/.

12 October 2004: Bud Schultz [West Aurora HS  Physics]           The Lightly Story 
Bud showed us a recording of the film The Lightly Story [http://www.answers.com/topic/lightly-story-in-a-darkened-room], which demonstrated transmission, reflection, and shadowing of electromagnetic waves.  The transmitter, with a frequency  in the GHz range, and antenna were of the type once used for radio communication with taxis.  The announcer (with a suspiciously Australian accent) showed that a receiver antenna had been connected in series with a small light bulb. When the length of the receiver antenna was appropriate (say, half a wavelength), the bulb would light when the transmitter was turned on and the receiver antenna was held near and parallel to the transmitter antenna.  By placing an identical antenna between it and the transmitter antenna, it was shown that the bulb did not light.  The intermediate antenna had produced a shadow, blocking the signal from the receiver antenna.  Furthermore, he showed that one antenna would reflect a signal to another antenna.  These phenomena of shadowing (lenses) and reflection (mirrors) are basic to all wave phenomena. But, where do we get such a transmitter?

Bud also showed us a rather intense Green Light Laser, which may be obtained for about $130 (including two batteries) at the Z-Bolt website at http://www.z-bolt.com/generic27.html.  This laser has a rated intensity of  4.99 mW -- just under the 5.00 mW level at which registration with the state of Illinois is required. Very interesting, Bud!