Light: reflection, refraction and colour

Light is a transverse wave that can travel through a vacuum at 300 000 km per second, the fastest speed there is. When light reflects from a mirror, the angle of incidence equals the angle of reflection, both measured from the normal, a line at 90° to the surface. When light passes from air into glass or water it slows down and changes direction: this is refraction. That is why a straw looks bent in a glass of water.

White light is a mixture of colours. A prism splits it into a spectrum: red, orange, yellow, green, blue, indigo, violet. Objects look coloured because they reflect some colours and absorb the rest. A red apple reflects red light. A filter lets only its own colour through.