A prism splits white light into a spectrum because each wavelength bends by a different amount.

The longer version

Glass slows light by an amount that depends on wavelength: short waves are held back more strongly than long ones. When a beam enters a prism at an angle, one face bends the rays and the opposite face bends them again, so a tiny difference in speed becomes a visible spread of colors on the wall.

Isaac Newton famously used a second prism to recombine the spread band back into white light, showing the prism was not coloring the light itself. Rock crystal and even a bevelled glass of water produce the same effect. Dispersion also explains why cheap lenses sometimes fringe bright objects with color.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.