“Seeing Sound”
Author: Dean Papadakis / Discrepant Event - Teacher's GuideSED 695B; Fall 2007
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Here is a picture of the embroidery loop with saran wrap tightly stretched over it and a tiny mirror glued to the saran wrap, along with a green laser pointing at the
mirror. The embroidery loop is placed in front of a radio speaker.
The discrepant event here is in the title, which suggests that we can “see” sound waves. However, we really can’t “see” sound waves. We can hear certain frequencies of sound. So in this demonstration, we are “transforming” the sound waves from a radio speaker, into light waves.
The Set-Up
· First you obtain a wooden embroidery loop and you either glue a sheet of saran wrap to the surface of the circular edge or you tighten the saran wrap in between the inner loop with the outer loop, using the set screw and clamp, as is done in the picture above.· Next you must obtain a tiny mirror, which can be cut from a larger piece of mirror using a glass cutter (or you can just break off a small piece using a pair of pliers). The mirror should be small (~ 3 mm) so that its mass is small and therefore its inertia is small.
· Next, you hang the embroidery loop with the small mirror in front of the speaker of a radio. Now set up a clamp and ring stand so that a laser pointer can be clamped into a position where it can be aimed at the embroidery loop.
· The mirror will reflect the laser light onto a wall or screen. When the radio is turned on, the sound waves will hit the saran wrap and cause it to vibrate, just like the head of a drum will vibrate when hit. As the saran wrap vibrates, so does the mirror and so does the reflected laser light and very amazing figures of light are produced. Of course, do this in a darkened room.
· Students will be amazed and entertained at the visibility of light waves being produced from sound waves. /
The discrepant event
Many Principles Are Ilustrated in This Demonstration· Light can be reflected off a mirror and obeys the “Law of Reflection”.
· Sound waves have energy that can cause something else to also have energy of motion (e.g. film of saran wrap).
· Wave transformations from transverse to longitudinal and back can be traced back to the original source. Radio wave entering radio is a transverse wave traveling at the speed of light. The sound coming out of the radio is a longitudinal wave traveling much slower at the speed of sound. The sound wave is causing a light wave from the laser to oscillate onto a screen. / The Principles
Standards
4. Waves have characteristic properties that do not depend on the type of wave. As a basis for understanding this concept, students know:
a. waves carry energy from one place to another.
b. how to identify transverse and longitudinal waves in mechanical media such as springs, ropes, and the Earth (seismic waves).
c. how to solve problems involving wavelength, frequency, and wave speed.
d. sound is a longitudinal wave whose speed depends on the properties of the medium in which it propagates.
e. radio waves, light and X-rays are different wavelength bands in the spectrum of electromagnetic waves whose speed in vacuum is approximately 3x108 m/s (186,000 miles/second).
Questioning Script
Prior knowledge & experience:
Ø Students should already have an understanding of how longitudinal waves differ from transverse waves.
Ø They should also know examples from the two categories of waves.
Ø Students should also understand the concept of reflection and specifically the “Law of Reflection”.
Ø They should also understand the idea that waves, whether they are longitudinal or transverse, can carry energy from one place to another.
Root question:
How can we see Sound?
Target response:
The way that we can see sound waves is by relying on the principle that sound waves have energy and therefore they can transport their energy to something else. They do this by traveling through some medium, such as air, as they leave the radio speaker, and they cause the saran wrap, which is in front of the speaker, to vibrate or oscillate due to the energy of the sound waves.
An additional variation to this demonstration can be done by, adding an additional laser of red wavelength. Now you have two sources of light, (green and red), which are slightly offset in where they reflect to the white board and you get a “Dueling Lasers Light Show”.
Common Misconceptions:
Ø The sound waves exiting a radio are transverse waves traveling at the same speed as light.
Ø We are listening to the “radio waves” when they come out of the radio.
Ø We can directly see “sound waves” with our eyes.
Ø Sound waves have no energy.
/ The embroidery loop is directly in front of the radio speaker, which is playing music.
/ This is an image of the white board, in a darkened room, which has the reflected laser light from the mirror in front of the radio speaker, showing wonderful patterns of light.
/ The varying light patterns are the result of different amplitudes of sound waves and different frequencies of sound waves causing the saran wrap to vibrate in different waves.
/ Sound waves of a person speaking, look different from the sound waves of music playing.
Different types of music will also look different.
References & Links:
I first saw this idea in a children’s book of demonstrations where a coffee can has a rubber balloon stretched over one open end of the can and then a small mirror is glued to the rubber balloon. The other end of the can is also open and placed in front of a radio speaker.