Lenses. Prism and rainbow

The scientific study of the colour of light was begun by Isaac Newton in 1666: when sunlight passes through a slit onto a triangular glass prism, a coloured band — the spectrum — appears on the wall. White light splits into seven colours: red, orange, yellow, green, light blue (cyan), blue, violet. The borders are not sharp, and seven is conventional. Different colours are refracted differently in the prism: red the least, violet the most; this is dispersion. With a second prism Newton recombined the colours into white light, proving white light is a mixture. White is also made by mixing red, green and blue light (Young): colour TVs and phone screens build pictures from these three colours. A rainbow appears because raindrops, like prisms, refract light, reflect it inside and split it into colours: it is seen when the Sun is behind us and the rain in front, with red on the outside of the arc. Bodies look coloured because they reflect some colours of the light falling on them and absorb the rest: a red ball reflects only red; a white body reflects all colours; a black body absorbs nearly all. A red object viewed through green glass looks black.