Light travels in straight lines, but it changes speed when it passes from one transparent material into another — for example, from air into water or glass. This makes the light change direction, or bend. The bending of light is called refraction.

Refraction is why a straw in a glass of water looks bent, and why a pool looks shallower than it really is. Lenses are curved pieces of glass or plastic that refract light. Magnifying glasses, glasses (spectacles), cameras and our own eyes use lenses to focus light.

White light, such as sunlight, is actually a mix of many colours. A triangular block of glass called a prism refracts each colour by a slightly different amount, so the colours spread out into a band called a spectrum: red, orange, yellow, green, blue, indigo and violet.

A rainbow forms when sunlight shines into raindrops. Each drop acts like a tiny prism: the light refracts as it enters, reflects off the back of the drop and refracts again as it leaves, splitting into colours.