Newton's Experiment Proving White Contains All Colors

Newton wasn't the first person to make a rainbow with a prism. Other people had already got the same result. But many of them thought the prism somehow 'coloured' the white light, like adding a dye. Newton's idea was quite different. He thought that white light was a mixture of all the colours, and the prism was just separating them from each other. He was right, and he proved it with a pair of neat experiments. First, he took his prism, as before, and stuck a narrow slit in the way of the coloured beams coming out of it, so that only one of them, say the red beam, passed through the slit. Then he put another prism in the path of this narrow beam of red light. The second prism bent the light, as usual. But what came out of it was only red light. No extra colours were added, as they would have been if what prisms did was add colour like a dye. The result Newton got was exactly what he expected, supporting his theory that white light is a mixture of light of all colours.

The second experiment was more ingenious still, using three prisms. It was called Newton's Experimentum Crucis, which is Latin for 'critical experiment' - or, as we might say, 'experiment that really clinches the argument'.

On the left of the picture above you see white light coming through a slit in Newton's curtain and passing through the first prism which spreads it out into all the colours of the rainbow. The spread-out rainbow colours then pass through a lens, which brings them all together before they pass through the second of Newton's prisms. This second prism had the effect of merging the rainbow colours back into white light again. That already neatly proved Newton's point. But just to make quite sure, he then passed the beam of white light through a third prism, which splayed the colours out into a rainbow again! As neat a demonstration as you could wish for. proving that white light is indeed a mixture of all the colours.

Using a prism to split light into a rainbow, he then used a lens to merge the rainbow back into white light and split it apart again.