The Moiré effect is a noticeable visual interference pattern that arises when two closely spaced, regular patterns.^[1] also known as shimmering pattern that appears when two similar repeating patterns overlap slightly out of alignment. Think of holding two window screens on top of each other and twisting them: suddenly you see large dark bands that belong to neither screen alone. The word comes from the French moiré, meaning "watered silk," a fabric made by pressing two layers together so their threads fall just shy of matching.^[2] 

Why it happens

The physics is surprisingly simple. When two grids of nearly identical spacing sit on top of one another, their lines periodically line up (creating bright zones) and periodically cancel out (creating dark zones). The result is a third pattern — much larger and coarser than either original grid — called a beat pattern, in direct analogy to the beats heard when two musical notes are close but not identical in pitch.^[3]  Mathematically, the resulting pattern is essentially the product of the two underlying gratings.

A small rotation between the grids produces circular or hyperbolic fringes. A tiny difference in line spacing produces straight bands. This sensitivity to misalignment is exactly what makes the effect both annoying (in photography) and useful (in engineering).

The moiré pattern isn't projected by anything. It exists only in the relationship between two layers — which is why you can't photograph "just one half" of it.

Where you run into it

Most people meet moiré accidentally. Digital cameras use a sensor covered with a regular pixel grid, and when a subject contains fine repeating detail — a checked shirt, a chain-link fence, city rooftops seen from a plane — the two grids interfere and produce swirling artefacts. Camera manufacturers fit anti-aliasing filters over sensors specifically to suppress this.

Printers face the same problem in reverse. Halftone printing converts images into dots, and if the dot screens of cyan, magenta, yellow and black plates rotate against each other, an ugly rosette pattern shows up on newsprint. Press operators carefully choose screen angles to keep those interference fringes too fine to see.

Turning a nuisance into a tool

Engineers learned long ago to exploit what photographers fight. In 1874, Lord Rayleigh suggested using moiré fringes for sensitive grating measurements, opening the door to an entire field.^[4]  Work by Ronchi, Raman and Datta in the 1920s, then Guild's 1958 book, turned fringe counting into a practical metrology technique.^[5] 

In modern optical testing, a grating is projected onto an object and viewed from a different angle; surface contours are recovered directly from the distorted fringes. The method handles surfaces too rough or too deviant for conventional interferometry.^[6]  The same principle measures strain in stressed materials, checks lens quality, and even animates textile design.