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Under light from the sun, a red object will scatter the red component of light, and absorb all others. Hence, the human eye perceives it as "red". White objects scatter all wavelengths of light.

Yesterday I bought an orange light bulb. Under its light, orange/reddish carrots look white. Why is that? A "white" object would have to scatter all wavelengths of light, but there's only red light to begin with.

Here's a photo of a person's hand, in white then in red light. Notice the color of the red nails. Do they appear white in red light, or does the white skin appear red?

Gorgeous red nails under red and white light

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    $\begingroup$ Under red light, a normally "red" object will appear RED... $\endgroup$
    – Semola
    Jul 27, 2014 at 12:08
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    $\begingroup$ Or put another way, in the presence of monochromatic red light, then all objects (likely to be encountered in everyday life, notably neglecting blackbody radiation) can only appear red, or some less intense red approaching black. It would be good to add a citation or context to your question stating where you got the notion that red objects in red light appear white. $\endgroup$
    – Phil Frost
    Jul 27, 2014 at 12:13
  • $\begingroup$ @PhilFrost: there's no citation, I just bought a colored light bulb and noticed the effect. $\endgroup$
    – Dan Dascalescu
    Jul 28, 2014 at 1:22
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    $\begingroup$ Do they appear white in red light, or does the white skin appear red? I see her nails red. Unless, you have a vision/monitor problem, I don't know why you ask this question. $\endgroup$
    – jinawee
    Aug 6, 2014 at 9:06
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    $\begingroup$ @jinawee: my comment was sarcastic (as I'm sure your hint at my vision problem was tongue-in-cheek). What you fail to realize is that a question author, by definition, doesn't exactly understand what underlying phenomenon is at the root cause of an observation, hence they may not be able to classify it as physics, biology, quantum optics, or whatnot. $\endgroup$
    – Dan Dascalescu
    Aug 6, 2014 at 10:02

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Under a red light, red objects and white objects look the same (bright), while blue objects still look different from white objects. Your brain does all of the decision-making that goes into "that tomato looks the same as that baseball, and that baseball is white, so the tomato must be white."

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  • $\begingroup$ Let's say I look at the tomato alone, through a peephole. There's nothing else white in sight. Won't the tomato still look white? $\endgroup$
    – Dan Dascalescu
    Jul 27, 2014 at 23:11
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    $\begingroup$ It's not an effect I've noticed. I have noticed that after reading for a while outdoors under a red light, coming indoors to a white light, everything has a distinctly greenish cast to it for a little while. So there may be some color-sensor exhaustion. That's more of a biology question, though. $\endgroup$
    – rob ♦
    Jul 28, 2014 at 0:28
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    $\begingroup$ I think the brain does some "automatic white balance adjustment" - picking the "whitest" object and referencing other colors from that. That, I believe, is how cameras do it. When the incident light is essentially monochromatic, this breaks down. $\endgroup$
    – Floris
    Aug 6, 2014 at 14:07
  • $\begingroup$ @rob You get a greenish hue? I get blueish. But yeah, as Floris said, that's because our brains are trying to correct for the colours and make whites appear white again $\endgroup$
    – Jim
    Aug 6, 2014 at 17:48
  • $\begingroup$ @Jim If it were blueish en.wikipedia.org/wiki/Afterimage would be a reasonable explanation, but since he said green, I was skeptical that the complementary color explanation could be the cause. $\endgroup$
    – jinawee
    Aug 6, 2014 at 18:41

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