Actually, the wave-particle duality turns out to be about the least special thing about light. So far as we know, all objects share this property, which becomes important for all small objects. Those include electrons, protons, neutrons, atoms, and even small molecules. There are more modern ways to think about the quantum effects shown by small things, so you actually can avoid using the wave-particle duality idea. We can describe more about that if youíre interested.
The speed of light in a vacuum appears to be constant. Light slows down a little in a medium, like water, due to the effects of the light waves emitted by the water moleculesí responses to the incoming light. In a vacuum, the temperature doesnít affect the speed of light, but it does affect how much thermal light is present.(Thatís the glow you see from anything hot.)
At very extreme temperatures, such as in the Big Bang, temperature might affect light. In fact, we donít even know if anything like ordinary space and time would exist at very high temperatures. But for the entire range we understand, the speed of light in a vacuum is constant.
(republished on 07/29/06)