When you look through one chain-link fence at another, you sometimes see a pattern of light and dark lines that shifts as you move. Moiré patterns are created whenever one semitransparent object with a repetitive pattern is placed over another. A slight motion of one of the objects creates large-scale changes in the moiré pattern. These patterns can be used to demonstrate wave interference.
Use a computer printer to make two transparencies using the pattern included below (click to enlarge).
CAUTION: Be careful with transparency material. If you decide to use a printer or copy machine, always make sure you have the correct transparency material. Otherwise, you may end up with a melted mess that can be fatal to your equipment.
Hold the two identical combs so that one is directly in front of the other and they are about a finger-width apart. Look through the teeth and notice the patterns of light and dark that appear. This is a moiré pattern. Slide the combs from side to side and watch the moiré pattern move. Now rotate one comb relative to the other and notice how the pattern changes.
If you only have one comb, hold it at arm’s length, about 1 inch (2.5 centimeters) from a mirror. Look through the comb at its reflection in the mirror. Notice how the moiré pattern moves when you move the comb to the side or slowly tip one end away from the mirror.
Look through two layers of window screen. Observe the moiré patterns as you slide one layer from side to side across the other, or when you rotate one layer. You can also create interesting patterns by flexing one of the screens.
If you only have one piece of screen, you can still make moiré patterns—even if the screen is still mounted in a window or a door. Have a friend hold a sheet of white cardboard behind the screen, and shine a single bright light onto the screen. (On a sunny day, sunshine can serve as your light source.) Start with the cardboard touching the screen, then move it away, tilting the cardboard a little as you go. The screen will form a moiré pattern with its own shadow. Replace the cardboard with flexible white paper and bend the paper. Notice how the pattern changes.
Use a computer printer to make two transparencies from the pattern of concentric circles provided in the Assembly section above. Hold the patterns up to a light source and look through them as you move them apart and together. The moiré pattern consists of radiating dark and light lines. You can also enlarge or reduce moiré patterns found in books and make them into transparencies using a copy machine. If you have an overhead projector, you can demonstrate moiré patterns for a large group by overlapping two transparencies on the projector.
When two identical repetitive patterns of lines, circles, or arrays of dots are overlapped with imperfect alignment, the light and dark lines that we call a moiré pattern appears. The moiré pattern is not a pattern in the screens themselves; rather, it is a pattern in the image formed in your eye. In some places, black lines on the front screen hide the clear lines on the rear screen, creating a dark area. Where the black lines on the front screen align with black lines on the rear, the neighboring clear areas show through, leaving a light region. The patterns formed by the regions of dark and light are moiré patterns.
In the case of the two sets of concentric circular lines, the dark lines are like the nodal lines of a two-source interference pattern. A typical two-source interference pattern is created when light passes through two slits. Along lines known as nodal lines, the peaks of the light waves from one slit and the valleys of the light waves from the other slit overlap and cancel each other. No light is detected along a nodal line.
In the black radiating lines of the moiré pattern, the black lines of one moiré pattern fill the transparent lines of the other. Note that as the patterns are moved apart, the dark nodal lines move together. This is the same thing that happens when light passes through two slits and the slits are moved farther apart.
Moiré patterns magnify differences between two repetitive patterns. If two patterns are exactly lined up, then no moiré pattern appears. The slightest misalignment of two patterns will create a large-scale, easily visible moiré pattern. As the misalignment increases, the lines of the moiré pattern will appear thinner and closer together.
Once you’ve learned to see moiré patterns, you’ll begin to see them practically everywhere. Look through two chain-link fences and notice the pattern. Watch it shift as you drive by. Look through a thin, finely woven fabric, such as a thin curtain or some pantyhose material. Now fold the fabric over and look again through two layers. You’ll see moiré patterns. Slide the fabric around and watch the patterns dance and change.