Wave Theory of Light


Part 1: Huygen’s Principle
Useful Resources:
Huygen’s Principle
http://www.colorado.edu/physics/phys1230/phys1230_fa01/topic14.html
Diffraction and Interference
http://electron9.phys.utk.edu/phys136d/modules/m9/diff.htm
Huygen’s Principle Animations
http://id.mind.net/~zona/mstm/physics/waves/propagation/huygens1.html

Use a wave tank on a desk over a large piece of white paper. Use the plane wave attachment (a long piece of wood) on the wave generator. Hook the wave generator up to a low voltage DC power supply. Set up a light source over the wave tank. Darken the other lights in the room. Observe what a plane wave looks like (you will see bands of light and dark on the paper underneath) and then set up obstacles as shown in the diagram. Start with a large space between the obstacles.





1. Sketch a plane wave with no obstacles.

2. Sketch the plane wave after it passes through a wide space.

3. Sketch the plane wave after it passes through a medium space.

4. Sketch the plane wave after it passes through a narrow space.

5. Using a physics book or the internet as a reference, state Huygen’s Principle and explain how this experiment illustrates it.


Part 2: Interference in a Wave Tank
alternate online simulation: Ripple Physlet
http://webphysics.davidson.edu/Applets/Ripple/Ripple.html

1. Connect two point source attachments to the waver generator. Put them at one edge of the tank. Draw and describe what you see.

2. Look at the diagram on the next page to help you understand what you saw in #1. The dotted lines represent troughs and the dark solid lines represent crests. Draw a large dot at all the points where a crest intersects a trough. With a marker, connect the dots with smooth curves. These nodal lines lie in regions where waves have canceled (destructive interference). Repeat this exercise by connecting all points where troughs and troughs or crests and crests intersect. Connect these points with a different colored marker. What do these lines represent?

Useful reference: "Interference in a Ripple Tank”
http://surendranath.tripod.com/Applets/Waves/RippleTank/RipTankApplet.html

Part 3: Young’s Interference Experiment
Alternate simulations:
Thomas Young's Double Slit Experiment
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/interference/doubleslit/index.html
Young’s Experiment
http://vsg.quasihome.com/interfer.htm


1. Obtain a plate with two thin slits cut into it. Draw what you would expect to see on a screen if you shone a light through these slits.

2. Put a red filter over the plate with the double slits and look through them at the light. The retina of your eye will be your 'screen'. Move the plate towards and away from your eye. Describe what you see. Is this different from what you predicted in #1?

3. Repeat #2 using a blue filter. Draw what you see. How is this image different from that of the one with the red filter? You can put a red filter over the top half of the slits and a blue filter over the bottom half so that you can compare the images simultaneously.

4. How do you think your results give evidence for the wave nature of light?

The following diagram shows the path of light waves in Young’s experiment.



5. Count the number of wavelengths between

slit A and point a: _____
slit B and point a: _____
slit A and point b: _____
slit B and point b: _____
slit A and point c: _____
slit B and point c: _____

6. When the number of wavelengths along each path is the same or differs by one or more WHOLE wavelengths, interference is constructive or destructive?

When the number of wavelengths along each path is the same or differs by a HALF wavelength (or odd multiples of a half wavelength), interference is constructive or destructive?

7. Using a physics book or the internet, summarize Young’s experiment and explain how it gives evidence for the wave nature of light.