Prism, dispersion and the rainbow

A glass prism has two triangular ends and three rectangular sides. Its two slanting faces are not parallel, so a ray bends on entering and bends again on leaving, and comes out turned through an angle: the angle of deviation.

Pass a narrow beam of white sunlight through a prism and a band of colours appears on the screen: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR). This band is a spectrum, and the splitting of white light into its colours is dispersion.

"Where did the colours come from?" They were in the sunlight all along. In glass, each colour bends by a different amount: red bends the least, violet the most. So the colours leave the prism along different paths and spread out.

Isaac Newton checked this. He placed a second prism upside down after the first. The colours went back together and came out as white light. So sunlight is a mix of all these colours.

A rainbow is a natural spectrum. After rain, tiny water droplets in the air act like small prisms. Inside each drop, sunlight:

• is refracted and dispersed as it enters;
• is reflected off the inside of the drop (internal reflection, not "total" internal reflection);
• is refracted again as it leaves.

The colours reach your eye from many drops at once.

A rainbow always appears in the direction opposite the Sun: the Sun is behind you. You can make one yourself on a sunny day by looking at a waterfall or a water fountain with the Sun behind you.