Why does an object which appears yellow in daylight appears red when illuminated with red light?

Question

physics

Answer 1

Objects reflect some colors and absorb others. The object you are referring to must be able to reflect  yellow and absorb some red color. So when the sunlight hits it a lot of yellow and a bit of red get reflected and we see it as yellow. If it is illuminated only with red only red gets reflected and we see it red.

Actually red and yellow are close on the spectrum. So most of everyday objects that reflect yellow reflect at least some red. It would be possible however, to have an object that would reflect only yellow and no red. Such an object would be black under red light.

Answer 2

When we "see" an object, what our eyes really see is light reflected off the object.  So if the light that is reflected is red, then the object will appear red.  It's also why in a dark room we initially see nothing at all, because there is no light to reflect.