GCSEC: Moiré Patterns

MOIRÉ PATTERNS

Watch a hanging moiré pattern as you move from side to side and front to back. You probably seem a moving interference pattern.

On the table top, line up a transparent pattern with its matching non transparent pattern so that they match exactly. Now slowly start moving the transparent pattern in different directions.

Moiré patterns result from looking through overlapping copies of an image. The top image must be transparent, the lower may or may not be. The moiré pattern results from an offset between the two images. When the two patterns are offset the dark of one image overlaps a clear area of the other, which interferes with the basic image. The pattern appears to move when there is movement between the two images.

Moiré patterns sometimes are visible in newspaper pictures. They are also visible if one looks through two sections of chain link fence, one behind the other, or through a pair of window screens.

A team of researchers looked at using two copies of an x-ray of a human heel to create a moiré pattern that would estimate bone loss due to osteoporosis.

Make moiré patterns by cutting holes in two pieces of cardboard and taping a piece of window screen or pantyhose over each hole. Or draw a pattern on a piece of paper and use a copy machine to make an exact duplicate on clear overhead transparency.

Hold two identical combs one behind the other and look through the teeth. Hold two layers of pantyhose material slightly separated and look through the pair. Hold two pieces of window screen slightly separated and look through the pair. Or hold any of these objects in front of a mirror. Finally, look through a chain link fence with another section of chain link fence in the background.

When you drop a pebble in a puddle of water, a series of rings move out from the center. Is this a moiré pattern? Why or why not?

There are a number of rings that form when a pebble is dropped into a puddle, and the rings are moving. However, there is no second or overlapping image so this is not a moiré pattern.

When you look at a moiré pattern on the table, it is possible to make the two images line up perfectly. When you look at a hanging moiré pattern, at most only part of the front image lines up perfectly with the back image. Why?

The two hanging images are about three quarters of an inch apart. When you look exactly horizontally and straight into the pattern, the two images line up perfectly. When you look above or below horizontal, the separation between the two images means that they do not line up.

Some astronomers study the strength of x-ray emissions from distant stars. X-rays are emitted in all directions from such stars, much like the rings that result when a pebble is dropped in a body of water. How could one of this exhibit's moiré patterns be used to predict the combined x-ray pattern from a pair of stars, each emitting x-rays? What if the pair of stars rotated about each other?

The x-rays would be a uniform emission in all directions from each star. The proper image would be the one with the same thickness rings. If the stars were stationary relative to each other, the interference pattern resulting from their joint emissions would be unchanging or static. The pattern could be modeled simply by placing the transparent copy of the image on top of the non-transparent copy with their centers offset. If the stars rotated around each other the interference pattern resulting from their joint emissions would be changing or dynamic. It could be represented by again offsetting the centers of the copies and then rotating the transparent copy in a circle.