Hear The Light?
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Jane F Long McAuliffe Elementary
1841 N. Springfield
Chicago IL 60647
The students will illustrate and describe a circuit which uses a tape
player, inductor, and flashlight to modulate a light beam.
The students will construct a flashlight transmitter using the materials
Modulated Laser from "Snack Book", 22 gauge wire, iron nails, flashlight
per group, two ear plugs per group, two alligator clips per group, batteries
and battery holders for two batteries and four batteries.
Step 1 A. Introduce topic to class by recording your voice into tape
recorder, then playing it back to students by way of the
modulated laser. (Teacher will have to construct this prior
to this lesson.)
B. Have tapes on hand with a variety of music from different
cultural groups and play this music to class by way of
C. Pass the tape recorder around and allow each student to say a
few words into the recorder. Play back to students by way of
D. Elicit questions and responses from students about what they
are seeing, hearing, etc.
Step 2 A. Explain the parts of the transmitter portion of the modulated
laser by starting with the tape recorder. Every student
should be familiar with a tape recorder.
B. Next explain that a simple ear plug is used. Be sure to
explain that the ear piece has been cut off, the wires
separated and the alligator clips attached. Pause to make
sure class is with you on each step.
C. Next explain that the tape recorder is connected to a wire
coil (induction coil). Most students will not know why so
you will come back to this part.
D. Point out the flashlight which all students should also be
familiar with. Explain that we have interrupted the path
between the batteries and bulb and rerouted the path through
the induction coil.
Step 3 A. Why do we need the tape recorder and flashlight connected to
the induction coil?
B. What have we recently studied that looks similar to this
C. What did we learn about electromagnets and how did we make
D. Suggest that if we make an electromagnet and change the
voltage or current going into that electromagnet, we will
also change the magnetic field. Pass out nails, wire,
battery holders and batteries to each group. Each group will
test how many paper clips can be picked up with two batteries
attached. Then test how many can be picked up with four
batteries. Did the magnetic field change? (Yes, it got
Step 4 A. Now that we see the magnetic field changes as the voltage
changes, what is changing the voltage in the modulated laser?
(the variation in the music or sound) Since music is made up
of different notes and each note is made up of a different
number of vibrations, the varying vibrations create a varying
push in current or voltage. This varying voltage changes the
magnetic field in the induction coil. What exactly does this
mean and why is this important for the working of this laser?
(In an induction coil, each loop in the coil interacts with
the magnetic fields produced by the other loops in the coil.
A self induced voltage is produced as this magnetic field
changes. Demonstrate that voltage changes when the magnetic
field changes by attaching a coil to a galvonometer and
passing a magnet through the coil).
B. The induction coil pushes the current towards the light bulb.
Since the music is changing the voltage going to the
induction coil, what must be happening to the light? (It is
also fluctuating or modulating)
C. Go over the meaning of the two terms: induction coil and
modulate. Now say that what we have just described is the
transmitter. This transmitter transmits or sends a signal to
Step 5 A. Hold up the solar cell and speaker/amplifier. This is our
receiver. In other words it receives the signal that we are
sending. Does anyone have any ideas about how this receives
the signal? (Solar cell changes the light energy to
electrical energy. Since the light beam is modulating, the
electrical impulses from the solar cell are also modulating.
As we described earlier, this changing electrical impulse or
changing voltage is going into our speaker and causing a
vibration in this speaker which is equal to the vibrations
from the music at the source. So what happens? We hear the
music that originated in the tape recorder from the receiver
across the room).
Step 6 A. Each group will make the transmitter part of the modulated
laser and test it on teachers equipment. Have a contest on
who can hear the signal from the furthest away.
Students will use the modulated laser they built as a guide and illustrate
the circuit involved in making this work. They will then describe in detail how
the modulated laser works.
5 points: Circuit is illustrated completely and accurately. The explanation is
clear enough for anyone to follow the instructions and build a
4 points: Circuit is illustrated as above but explanation lacks clarity.
3 points: Circuit is illustrated but not labeled, and explanation lacks clarity
2 points: Circuit is not illustrated completely and explanation unclear.
1 point: Illustration and explanation lack details necessary to understand the
Have various tapes from different cultural backgrounds to play for the
students. Also ask the students how this lesson can apply to our lives and our
future. What are the future uses and applications of this type of communication
Doherty, Paul, The Exploratorium Science Snackbook, Exploratorium, 3601 Lyon
Street, San Francisco, CA 94123, 1991.