No assembly needed.
(15 minutes or more)
Hold 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 moire pattern. Slide the combs from side to side and watch the moire 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 cm) from a mirror. Look through the comb at its reflection in the mirror. Notice how the moire 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 moire 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 moire 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 moire pattern with its own shadow. Replace the cardboard with flexible white paper and bend the paper. Notice how the pattern changes.
Use a copy machine to make two transparencies from the pattern of concentric circles provided with this Snack as a separate page, here. Look through these two patterns as you move them apart and then together. The moire pattern consists of radiating dark and light lines.
You can project moire patterns so that a large group can see them. Just make two transparencies of a repetitive pattern and overlap the transparencies on an overhead projector. Moire patterns from books may be enlarged or reduced and made into transparencies on a copy machine.
When two identical repetitive patterns of lines, circles, or arrays of dots are overlapped with imperfect alignment, the pattern of light and dark lines that we call a moire pattern appears. The moire pattern is not a pattern in the screens themselves, but rather 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 moire 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 moire pattern, the black lines of one moire 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.
Moire patterns magnify differences between two repetitive patterns. If two patterns are exactly lined up, then no moire pattern appears. The slightest misalignment of two patterns will create a large-scale, easily visible moire pattern. As the misalignment increases, the lines of the moire pattern will appear thinner and closer together.
Once you have learned to see moire 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 white handkerchief, or some pantyhose material. Now fold the fabric over and look again through two layers. You'll see moire patterns. Slide the fabric around and watch the patterns dance and change.
If your browser uses Shockwave, watch moire in motion at our Spatial Beats phenomenon page.