Science Questions

Why do colours look different under a sodium streetlight?

Sun, 8th Jan 2012

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Question

Why do different light sources look different under a sodium streetlight? asked:

I've got a pram and the hood and the apron of the pram are green. But when I run the pram underneath one of the original sodium streetlights the hood and the apron turn white! Why?

Answer

Dave - That's to do with the way your brain is attempting to interpret the colour information.  Your brain attempts to take into account that different light sources actually look very different. 

If you have ever seen a photo taken inside with a camera which is an attempt to compensate for this inside with normal incandescent bulbs, it looks incredibly yellow and your brain immediately tries to compensate for that.  What you're actually seeing is the whole thing is actually looking different types of orange, different points of orange because there's only one colour.  But your brain is attempting to compensate for the fact that light is very orange and because everything looks orange it assumes that anything is reflective of orange is probably white and anything which isn't is probably dark. And so, you kind of interpret it as white because your brain is compensating.

Chris -   I got asked something similar by some guys in South Africa about why when you look at ordinance survey maps which have red roads on them for example, if you look at them under a red torch, why the red roads don't look a weird colour and that's the same sort of question, isn't it Dave?

Dave -   In fact, if you look at a red roads under a red light, they disappear entirely. White reflects red and so does red, so they both look the same.

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