# Given is an object which is cyan when illuminated with white light. What is the perceived colour of the object when illuminated with yellow light?

I know the perceived colour of an object depends on which wavelengths of the incoming light are reflected and not absorbed, which lead me to believe that it is impossible to be red or yellow... I think it should be green but I am not sure because I am having trouble understanding this...

• It depends on both the spectrum of the illumination and the spectral reflectance of the object. It could look yellow or be completely black or even be somewhat red-ish. There is no simple answer. – CuriousOne Apr 12 '16 at 9:09
• @CuriousOne Thanks for reply, will try to think more about it... – phantom Apr 12 '16 at 9:17
• look up color perception too, it is a biological response of the retina of the eye and there are graphs. one can get colored pictures with only two frequencies of light, and those close by . intropsych.com/ch04_senses/lands_demonstration.html – anna v Apr 12 '16 at 10:34

Simply, if you have an object that appears Cyan in White light, then that must mean it absorbs Yellow and Magenta. If you then illuminate that object only with Yellow light, it will appear Black (dark) because the object absorbs Yellow light.

• Very misleading. Cyan, yellow and magenta are subtractive colors. They aren't "absorbed". The other answer gives a correct explanation. - 1 – LLlAMnYP May 20 '17 at 15:31

White light is composed of red + green + blue.

Cyan = blue + green

Magenta = red + blue

Yellow = red + green

So if an object is cyan when illuminated by white light, it means that it reflects blue + green light, i.e. the object does not reflect red light.

When the same object is illuminated with a yellow light, it is essentially being illuminated with red + green light. As mentioned, the object does not reflect red light, and so only reflects the green light.

Therefore, the object is perceived to be green in yellow light.

• Yellow illumination could be a mix of red and green or it could be a pure yellow wavelength or some other combination. Your answer assumes it's red and green. – Brandon Enright May 20 '17 at 15:38