Why don't we see rainbows under street lights?

08 January 2012



Dear Naked Scientists,

My 7 year old daughter asked me this evening, why we don't see rainbows underneath street lights when it is raining?

I suggested it was a combination of the wrong angle and the lights not being bright enough. She wasn't convinced by my answer and insisted I ask someone intelligent for the real answer.

Please could you enlighten us.

Thanks very much,

Matthew Taylor


Dave - Okay. There are probably two effects with this. One of them is that a streetlight is incredibly less bright than full sunlight. I just did a quick calculation. I think it's a thousand times less bright than full sunlight and something you might have noticed when the light gets very, very dim is that it's very hard to pick out colours because your eyes are much less sensitive to colour when the light is very dim. So even if there was a rainbow there, you wouldn't be able to see it very well. And with some kinds of streetlight, they will actually create a bit of a rainbow. You'd have to be looking away from the streetlight, but it would be so dim, you wouldn't be able to see it.

But there is a second effect which is that a lot of streetlights that have all the colours of the rainbow in them, especially the very yellowy ones, they've actually effectively only got one colour in them, a kind of just pure orange colour. The reason why you can see a rainbow normally is because white lights got all the colours of the rainbow mixed up in it, so the rain could split them up and you can see them in different places. But if the light has just got orange in it, the raindrop can't split it up so all you'll see is orange, so you won't get the colours of the rainbow. Chris - Effectively you get an orange rainbow. Dave - Yes, we just get an 'orangebow' rather than a multicolour one.

Chris - You did an experiment on that for the Crisp Packet Fireworks Book in a Kitchen Science we did a long time ago using your car indicator and a streetlight to show this sort of effect, didn't you?

Dave - That's right. If you look at any kind of brightly coloured thing under yellow streetlights, it just looks kind of gray. Actually, it just looks like orange and black. There's no colour there at all, but then if you take it to normal or sun's normal light, or just look under white torch, it looks all the colours of a rainbow, and even if you look at it in an orange indicator light, quite often, you'll see some colours of the rainbow in there because it's a mixture of green and red, and yellow. So you'll see some colours but under a streetlight, you're going to only see the one.

Chris - There's more than one way to make orange. Dave - There's so many ways in making orange.


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