The kaleidoscope is an optical device consisting of mirrors that reflect images of bits of coloured glass in a symmetrical geometric design through a viewer. The design may be changed endlessly by rotating the section containing the loose fragments. The name is derived from the Greek words kalos ("beautiful"), edos ("form"), and skopen ("to view").

The kaleidoscope illustrates the image-forming properties of combined, inclined mirrors. If an object is placed between two mirrors inclined at right angles, an image is formed in each mirror. Each of these mirror images is in turn reflected in the other mirror, forming the appearance of four symmetrically placed objects. If the mirrors are inclined at 60 degrees, a hexagonally symmetrical pattern results from one object producing six regularly placed images.

 
A simple kaleidoscope consists of two thin, wedge-shaped mirror strips touching along a common edge, enclosed in a tube with a viewing eyehole at one end. At the other end is a thin, flat box that can be rotated; it is made from two glass disks, the outer one ground to act as a diffusing screen. In this box are pieces of coloured glass, tinsel, or beads. When the box is turned or tapped, the objects inside tumble into an arbitrary grouping, creating a striking symmetrical pattern. The number of combinations and patterns is effectively without limit.