The Spectrum of White Light

Sometimes, if it rains and the Sun shines at the same time, you may see a rainbow. You have to stand with your back to the Sun; you will see an arch of beautiful colors looking as though it is hanging in the air.

Splitting Light

You can see the colors of the rainbow for yourself by sending a ray of white light into a glass prism. (A prism is a triangular glass block).

When the light enters the prism it bends-it is refracted. It also bends as it leaves the prism.

Something else happens. The white light is split up into a spectrum of colors. They are the same colors as you see in a rainbow. The splitting up of white light into separate colors is called dispersion.

The colors of the spectrum always appear in the same order:

Red Orange Yellow Green Blue Indigo Violet

(Indigo is a dark blue-purple color). Although we say there are seven colors in the spectrum, there are no dividing lines where one color changes to the next. The color changes gradually from one shade to the next.

Explaining Dispersion

Because a spectrum appears when light passes through a glass prism, some people imagined that it was the glass that gave the colors to the light. Isaac Newton realized that this was incorrect. He showed that while light (such as light from the Sun) is a mixture of all the different colors of the spectrum.

Dispersion happens because of refraction. When white light enters a block of glass, some colors bend more than others. Violet bends the most, red the least. This means that the different colors travel off in different directions, and so we can see them separately.