People can see moire patterns by looking through two layers of something that has a grid. For example, looking through two screen doors at once can produce the moire effect. For the effect to appear, the two patterns meet one of two criteria. Either they must be identical but offset by a significant angle between 20 and 45 degrees, or the two grids must be of different sizes.
People can create moire patterns by drawing two identical sets of concentric circles and then slowly sliding one set atop the other. As the curved lines intersect, various moire patterns form. Another way to construct a moire pattern is to draw two identical sets of parallel lines and lay one set atop the other at a perpendicular angle. At this point no moire pattern will appear, but rotating the top image slowly while the bottom image remains still produces moire patterns whenever the sheets are at angles between 20 and 45 degrees with respect to one another.
Most people who construct moire patterns limit them to black and white images. However, it is also possible to make moire patterns using curved lines drawn in any color. When the two patterns used are different colors from one another, the fringes or ripples produced by superimposing them will be of a third color.
Physicists use moire patterns in diagrams to represent potential electromagnetic fields and fluid flow. They are also useful in order to solve problems in a variety of subfields in physics, including stress analysis, optics and wave motion. Psychologists use moire patterns to analyze visual perception in subjects.