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Question:
When I shone a blue tinted daylight bulb into a light blue object, it appeared white. Is it possible to use certain light filters to make objects appear colorless or white, or is it a trick of the eye? if so, how does this work?



Replies:
It is not a trick of the eye in the usual sense. The "color" your eye sees is composed of the wavelengths of light absorbed or reflected by the object. In addition, the amount of light of a certain wavelength depends upon the wavelengths that the light source produces. It is the combination of these two effects (plus the eye's sensitivity to different wavelengths of visible light) that results in the "color" one perceives. Technically, this effect is called "metamerism". You have to keep in mind that the color of an object appears under reflected light by the complementary wavelength that is absorbed by the object. So for example, an object appears "yellow" because the object removes "blue" from the wavelength that is incident upon the object. In any particular case the perceived color becomes a rather complex response to the wavelengths absorbed and/or reflected.

Vince Calder


It is a trick of the eye. Since the light incident on the object is enhanced in the blue, it is reflected fairly evenly throughout its frequency range by the light blue object. Your eye interprets this to be a white object, because that is what white objects do.

Your eye makes this compensation because we see things in light of varying compositions: the red of sunset, the blue of shadows, the yellow of daylight, and the orange of firelight. At sunset, a zebra does not turn red, so our eyes and brain do the best they can to relate what they see to what is probably there. In the case of the pale blue object in bluish light, it does not work. But you really cannot do any better than that; in pale blue light, a white object reflects pale blue light, so the pale blue object looks just like a white one!

Richard Barrans
Department of Physics and Astronomy
University of Wyoming



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