Toby Heine, of Nelson Central School, asks :-

Recently I saw a rainbow at night. Are these common?

John Campbell, a physicist with an interest in atmospheric optics, responded.

Rainbows at night are not often observed but they are there if you know when and where to look.

A rainbow is the bright, 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. The outside of the bow appears iridescent red with the other colours predominating in turn through to violet on the inside of the bow.

At night a bright moon (ie sunlight reflected from the moon) can be the light source. The conditions needed are exactly the same as for a rainbow produced during daylight. All you need is to stand with your back to the moon when moonlight is falling on raindrops and you will see a `white' rainbow. Only about a tenth of the sunlight falling on the moon is reflected so night bows are much weaker than day bows. We dont see a coloured bow at night. Our eyes dont register colour in weakly reflected light at night because our colour receptors are too inefficient to be effective at low light levels.

Rather than wait for rain get used to producing a rainbow using the spray from a hose on a sunny day. Then do the same on a bright moonlit night and you will see the bow.