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A blue object appears black when illuminated with which color of light?

A. Green
B. Yellow
C. Cyan
D. Blue
E. None of the above

Answer: B. Yellow

Justification: A blue object illuminated with yellow light appears black because it absorbs the yellow light and reflects none.

I marked E... it seems to me that if the object is perfectly blue, then green or yellow are equally correct, whereas if the object is imperfectly blue, E is the most correct, and only one answer could be chosen...

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  • $\begingroup$ Given the multiple choice nature of the question I assume it was given in a fairly low-level class. In that case the official answer could be "right" in the sense that it matches what was taught, but it still wouldn't be right in any absolute sense as it takes far too simplistic a model for both reflective behavior and color perception (a failing shared by all the existing answers). $\endgroup$ – dmckee --- ex-moderator kitten Sep 9 '19 at 21:07
  • $\begingroup$ It was in a Physics GRE study book. $\endgroup$ – DJG Sep 9 '19 at 21:33
  • $\begingroup$ You would be expected to make assumptions on normal conditions, like you would expect the light to be strong enough to where your eyes would make out more than grey, and thus the tail of the green part would be enough to see the blue part of it. $\endgroup$ – user234190 Sep 10 '19 at 1:28
  • $\begingroup$ @dmckee, indeed. In my opinion, this is a problem. When you know a LOT of physics, and you encounter a question like this, you start second guessing yourself because you don't have enough contextual cues in the question to know what level of answer the questioner is looking for. $\endgroup$ – David White Sep 10 '19 at 2:56
  • $\begingroup$ @David White it's really not that complicated. People are expected to answer practical questions or else what is the point if one can't apply the basics of what one learns. There are other similar questions. Taking things beyond first order is often ignored. Like you could say yellow light could make it appear non black if it were a strong enough yellow laser, for various reasons. $\endgroup$ – user234190 Sep 10 '19 at 3:56
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I suppose that a cyan object reflects both blue and green light and so it doesn't appear black in blue light.

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I would say that after looking at a BBC Bitesize article on this subject to refresh my memory, that any other colour of light besides blue radiated onto this blue object would cause the object to appear black. Let me explain: if an object appears blue then that means it absorbs all other colours of light and reflects the blue light. If there is no blue or white light containing blue light in its spectrum then the object will appear black as there is no blue light to reflect.

That means the answer to the question should be: all colours except blue and white. I think the question is incorrectly asked.

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  • $\begingroup$ Cyan light is not monochromatic but contains blue light. $\endgroup$ – Pieter Sep 9 '19 at 20:29
  • $\begingroup$ Yes, I forgot about composite colours in my comment above. Thanks for the correction. $\endgroup$ – happyspartan Sep 10 '19 at 13:13
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The definition of a color of an object, as I see it, is the wavelength of light that it reflects. If an object is truly blue, that means that it only reflects the wavelength of light that we see as blue, and nothing else. Therefore, the question is incorrectly phrased as the object will appear black under all light except blue (and white, since it consists of all the visible wavelengths.)

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