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# Wave Interference

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Source:
Author: Marc Rosner

Interference is a property of light that can be observed when different sets of light waves mix together. Examples of interference include the rainbows you see in soap bubbles, the colorful spectrum of opals, and the shimmering colors of some bird feathers. In some regions of interference patterns, the waves of light are in phase, with matching peaks and troughs, creating bright zones. In other regions, they are out of phase, with opposing peaks and troughs, creating dim zones. There are a variety of ways to demonstrate interference, both as bright and dim regions, and as different colors representing the different wavelengths of light.

Interference results from overlapping waves. When two wave peaks (high points) meet, they combine into a larger wave. When a wave peak and a wave trough (low point) meet, the waves cancel each other out. The positions of peaks and troughs are called phases.

### Materials

• shallow baking pan
• water
• chopsticks or 2 pens and a rubberband, or a Y-shaped object
• some or all of the following: bubble solution, opal jewelry, feathers, two 20-cm-square pieces of cut window glass with edges finished. (A hardware store can cut glass to order. Ask them to sand the edges for you to make it safe to handle.)
• various light sources (bulbs, laser pointer, lab laser, ordinary flashlight, flashlight with lens)
• convex lens
• sheet of paper or dollar bill
• 2 stands with clamps
• white screen or wall
• scissors
• transparent tape
• black construction paper
• black spray paint
• thumbtacks
• one-hole paper punch
• leather punch
• ball bearing

### Procedure

Vary the effect by creating a small gap between the plates at one edge, using a piece of paper or a dollar bill.

Alternatively, substitute a convex lens from an old pair of glasses or broken camera for one of the pieces of glass, and examine what happens when you press the lens against the glass.

To fashion filters with different holes, take some old, unneeded slides and darken them with black spray paint. Then punch different size holes in them, using thumbtacks, a paper punch, and a leather punch.
1. Fill a shallow pan with water. Fasten two chopsticks together with a rubber band to make a two-pronged disturbance source. Tap the water with the chopsticks (or tap two fingers, two pens, or a V-shaped object) and observe the patterns that result when waves from the two disturbances intersect. Where waves combine, you get larger waves (constructive interference). Where they cancel each other out, you get calm spots (destructive interference).
2. Examine a variety of materials where optical interference causes patterns of light and dark, or colored patterns. You can examine soap bubbles, opals, and bird feathers. If you live near a marina, you can observe the water where a drop or two of oil has spilled from the boats—colored rainbows are emitted from the expanding layer of oil as it spreads across the surface. You can create optical interference patterns by sandwiching two thin, smooth plates of glass together. Aim a light source at them, and press them together. Shimmering rings of light and dark, with colored hues, become visible.
3. Use two stands with clamps to set up a laser pointer so that its beam passes through a slit filter onto a white screen or wall. (You can make a slit filter with an old photographic slide. Cut out the plastic image, and tape two black paper rectangles in its place, close together, leaving a uniform slit running vertically down the middle.) In one of the great mysteries of light, the beam passing through the filter magically creates regions of light and dark called a diffraction pattern. Replace the slit filter with other types of filters: slits of different widths and shapes; holes of different sizes; a lens in front of a ball bearing.
4. You can change certain variables in your setup to create a more formal experiment. Vary the light source to compare the differences between a laser pointer, a lab laser, an ordinary flashlight, a flashlight with a lens, and bulbs of different types. Or study the diffraction pattern on the screen as a function of distance between the laser and the filter, and/or the filter and the screen.

References

Awesome Experiments in Light and Sound by Michael Anthony Dispezio (New York: Sterling. 2000).

Making Waves: Finding Out about Rhythmic Motion (Boston Children's Museum Activity Book) by Bernie Zubrowski (New York: Morrow/Avon, 1994).