The kaleidoscope is a tube of mirrors containing loose coloured beads or pebbles, or other small coloured objects. The viewer looks in one end and light enters the other end, reflecting off the mirrors. Typically there are two rectangular lengthways mirrors. Setting of the mirrors at 45 degrees creates eight duplicate images of the objects, six at 60 degrees, and four at 90 degrees. As the tube is rotated, the tumbling of the coloured objects presents the viewer with varying colours and patterns. Any arbitrary pattern of objects shows up as a beautiful symmetric pattern because of the reflections in the mirrors. A two-mirror model yields a pattern or patterns isolated against a solid black background, while a three-mirror (closed triangle) model yields a pattern that fills the entire field.

Invented by Sir David Brewster in 1816 while conducting experiments on light polarization, it was patented in 1817. The initial design was made from a tube in which Brewster placed pairs of mirrors at one end, and pairs of translucent disks at the other end. Between the two, he placed the beads. Initially intended as a science tool, it was quickly copied as a toy.