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I was reading this blog post: http://ianstormtaylor.com/design-tip-never-use-black/

Which phenomenon explains this:

But I must have been thinking the same thing, because one of those days in art class Mrs. Zamula came in with a blue light bulb to prove it. She screwed the bulb into a clamp-light, plugged the light in and clamped it to a stool. Then she got a pure-white ball from her cabinet full of pure-white things (that she kept for figure drawing exercises) and placed it on a pure-white pedestal, under the light. And sure enough, when she turned on the blue light, the shadow cast by the ball was an orange tint, not black.

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More a question about human vision than a question about physics. You might get an answer though, which probably will take into account the processing in the human brain compensating the light balance towards white. – Johannes Aug 10 '13 at 11:14
    
+1 for the use of "grok" in that link. – Emilio Pisanty Aug 10 '13 at 11:51
    
It doesn't. We merely see the rest of the areas lit up by blue and so we percieve the part which isn't, to be the complementary colour of blue - which is orange. – mikhailcazi Aug 10 '13 at 12:35

The color of the object casting the shadow is irrelevant. What this demonstrates is that our eyes perceive color partially in relative terms.

When your eyes look at the general area illuminated by the blue light, they start to cancel out the blue color. This is a lot like a camera sensing ambient light to determine the white ballance. You still see blue because not all the blue is cancelled out. However, the small shadow will also have the presumed ambient blue subtracted from it, which makes it look orange.

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Is there a simple experiment that could help me see this phenomenon, which doesn't imply having a blue light and a pure white object as in the article ? – alecail Aug 10 '13 at 15:10
    
@alecail You can recreate the illusion with a dark room, a white card, a white light bulb, a colored bulb, and your fingers. Go in the room, turn off all lights except the two, hold the card where both lights shine on it, and make shadows with your fingers. Where your fingers block only the white light, the shadow, will have the color of the colored light. Where your fingers block both lights, the shadow will be black. Where your fingers block only the colored light, then the shadow actually will be white, but you probably will perceive it as the color complementary to the colored bulb. – james large Apr 6 at 22:16

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