Waves in Water, Sound and Light

`What Is Light' WISE group -- Fall 2002

 

What's a Wave?

In WISE this year our group decided to research waves. Our group split up into three major categories. We were assigned to explore the role that waves play through sound, light, and water.

We'll start out by demonstrating three kinds of waves: longitudinal (push-pull); transverse (up-down); and shear (twisting), with some HUMAN WAVE DEMOS.

Waves can Move, Reflect and INTERFERE

We demonstrate waves with the snake spring in the aisle ....

Sound Waves

A sound wave can be described as a disturbance carried through a medium from one location to another. In our demonstration, we placed two tuning forks across from each other and used a mallot to hit the forks. We also used a slinky to see how a sound wave behaved.

By doing this, we were able to hear the sound waves and the interference of the waves. When the tuning forks are of similar frequencies, they create constructive and destructive interference. You can hear the beats of the two forks in step and then when they go out of step. When the beats are in step, you hear a loud "wooing" sound and as they go out of step, the sound quickly diminishes. This pattern continues to repeat. We also investigated the effects that the sound waves of one fork have on the other. To do this we hit one tuning fork and noticed that the other tuning fork began to vibrate. Then we stopped the initial one that was vibrating and noticed that the other fork was vibrating by itself. This was the result of sympathetic resonance. The sound waves of the fork we hit were affecting the other fork and causing it to vibrate.

A sound wave works by compression and decompression of particles and is defined as a longitudinal wave. In order to see the longitudinal waves, we used a slinky to carry out our demonstration. We had one person on each end of the slinky and we stretched it out from end to end. To create a pulse, one person had to vibrate the first coil up and down. After doing this, a wave could be seen as it travelled across the slinky, bouncing off one end and returning. The longituinal wave could be seen as a push, similar to a chain reaction, in which one ring pushes against the other creating a ripple. This ripple continued back and forth until the wave died out.


Waves in Water (Ripple tank)

Our specific section that we researched was water. A water wave is a wave that oscillates up and down, or side to side. This type of wave is defined as a transverse wave. Consider this, If a ball were floating on the surface of the ocean, would it travel in to shore on a wave or would it merely float in one general area? A wave is only a disturbance of matter, not a displacement of matter. The ball is not carried in by the wave, it merely moves up and down on top of the waves as they go by.

To explore the actual movement of a wave, we set up a ripple tank. A wave interference is a phenomenon which occurs when two waves meet while traveling along the same medium. The interference of waves causes the medium to take on the shape which results from the combined effect of the two individual waves upon the particles of the medium. Through this, we learned about the constructive and destructive interference of waves. Constructive interference occurs when the crests and the troughs of each wave match up exactly. When this happens, the amplitude of the resulting wave is both amplitudes added together. This forms a third wave which is twice as great in magnitude. A destructive interference occurs when the crest of one wave matches up exactly with the trough of another wave or vice versa. When this happens, the two amplitudes cancel each other out and the resulting amplitude is zero.

We can also demonstrate interference of water waves by overlapping two sets of circles printed on transparencies, on an overhead projector. The effect will be something like the picture shown on Doug's web page, but it will be possible to change the separation of the two sets of circles and demonstrate the changes in the interference pattern.

Light Waves

Just like sound, light is made up of waves that travel through a hard to see medium, the air. These waves are made up of ripples in electric and magnetic energy in space. To demonstrate the size of wavelengths, we can compare it to the width of a strand of hair. The width of a strand of hair is 1/10 of a millimeter, and the length of a wave of light is about 200 times smaller then that. Light waves which we can see are called visible light. The size of the wavelengths depend on the color of the light.

To demonstrate some properties of light, we will use Rainbow Glasses. Rainbow Glasses are made up of a sheet of plastic with many microscopic punctures. When a laser is shown through the glasses, the light goes through the hole and spreads out in all directions. This is called diffraction. The light coming from different holes re-combines, which is called interference. In some places, the interference is constructive. This is what we perceive to be a single dot on the wall. Just the opposite happens when the glasses are worn by someone. The light from the dot on the wall hits the glasses and is diffracted and re-combined towards the eye. When a laser is shown through the glasses,or worn, a pattern of numerous dots appear on the wall. Furthmore, when two or more glasses are held together and a laser light is shown through both of them, there are more dots and the pattern becomes more complex.