Prisms and lenses refracting three rays of light. From left to right, there is a diverging biconcave lens, a converging biconvex lens and two triangular prisms. The rays start parallel and diverge after passing through the first lens, then converge after passing through the second. The rays are then brought to a focus by the two prisms. Lenses change the path of a ray of light through refraction, which occurs when light passes from air to glass, due to it travelling more slowly in glass.