Georgie Anderson, of Rangi Ruru School, asks :-

Why is a rainbow bow-shaped and does it go round in a complete circle?

John Campbell, a physicist at the University of Canterbury who has an interest in light and colour in the atmosphere, responded.

All rainbows are the same angular size although we see more of the arc if the Sun is low in the sky.

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.

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.

It is a bow because there is the same angle between the ray from any of the water droplets and the line from the Sun through our eye.

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. If the Sun is low in the sky we see a larger arc of the rainbow than when the Sun is higher in the sky. If we stood on a mountain peak at sunrise or sunset looking into distant rainclouds we would be able to see more than half of a circle.

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, preferably against a dark background. You will see it as a bow whose centre is the line between the Sun and our eye. Using the hose to produce waterdrops not far from your eye, and a low Sun, you see the whole circle of the rainbow.