Rebecca Julian, of West End School, asks :-

How are rainbows formed?

John Campbell, a physicist with an interest in light and colour in the atmosphere, responded.

A rainbow is the coloured bow we see whenever raindrops are illuminated by the sun behind us.

When a ray of white light from the sun enters a spherical droplet of water it travels through the droplet, reflects off the rear surface and re-emerges from the front surface. If we look away from the sun we see a circular bright bow that appears to be at the same angle across as that formed by about four hand spans at arm's length.

Therefore a rainbow is not something solid in the air. The light from the sun has come to our eye by being reflected from the inside of a drop of water hence it appears to come from the direction of the raindrops which are usually a long way away.

The colour display of the bow comes about because in water blue light travels slower than red light so a blue ray is bent through a larger angle than the red ray. Hence the outside of the bow appears iridescent red with the other colours predominating in turn through to violet on the inside of the bow.

Rather than wait for a rainy day I suggest that you and your class-mates spray water into the air using a garden hose and, with the sun behind you and low in the sky (hence do this early or late in the day), look for the rainbow. If you do this against a very dark background you may see a second bow outside the main bow. For this secondary bow the light has been reflected twice inside the raindrop.