I've been wondering about this lately. We've most likely all seen the basic example at school of projecting three colours of light at a screen and noticing that the area where the colours overlap turns out to be white. But what I'm wondering is whether there is actually any physics to that. Does the fact that it looks white to us actually have anything to do with properties of those specific wavelengths of light, or is it merely that our eyes have receptors for those three colours? From what I understand, true white light is made up of all the different colours of the visible spectrum, so how could three single wavelengths produce the same effect physically?