Christopher Moore is an Assistant Professor of Physics at Longwood University in Farmville, VA.As students of physics, we learn when studying Special Relativity that the speed of light is constant. When we study geometric optics, we learn that the speed of light slows down in a medium. This seeming contradiction leads to a completely understandable misconception about the nature of light.
I was reminded of this while reading this forum post. One of our resident crack-pots asks the following questions:
Does a smaller light speed in a medium rule out the wave theory of light?
I guess that depends on how you define “wave theory,” but Huygens’ and Fresnel’s theories certainly could account for varying speeds in a medium, much like wave theories of sound. However, we now know that light is not really a wave, nor is it a particle. It is a wavicle we call a photon. More on that here. And yes, the quantum theory very accurately describes a slowing of light’s average speed in a medium. Note the emphasis on average!
The quantum theory (an electromagnetic theory) tell us the speed of light is indeed constant, no exceptions. It turns out, individual quanta of light propagate at one speed. That’s it. Wavicles/photons of light move at c and no faster or slower. So why does light appear to slow down in a medium, such as water?
Light propagates slower through a medium since the photons are absorbed by the atoms in the material, then re-emitted some short time later. Light always moves at a constant speed. However, when it is incident on a material, it is transformed into an excited energy state — that is to say, it ceases being a photon. Once the excited atom goes back to its lowest energy state, then the excess energy is emitted as a photon. This process takes time.
Think of it this way. A photon hits a particle. The particle gobbles up the photon. The photon is no longer a photon; it is now energy the particle uses to do jumping jacks. After a short period of jumping jacks, the particle collapses onto the sofa. Energy being conserved, the particle spits out a new photon. This all takes some time, which is why the speed of light appears to propagate slower in a medium.
The quantum theory has all of this stuff within it. It fits VERY well. The 400 year old wave theory says nothing of absorption and excited states, but it still predicted slower average speeds in a medium, it just failed on other tests.