Why does the velocity of light appear to decrease in denser media?

Answer 1

Light's speed is a constant, but it appears to slow down in any media. Everything through which light might have to travel will cause it to appear to slow down. And it isn't necessarly associated with the density of a medium, but its refractive index. Let's set that aside and tackle the question. Why does light appear to slow down in a medium? Let's look at the medium and how light interacts with it. Air, glass, water, plastic or anything else is composed of atoms or molecules. These structures, though they are so small we almost cannot directly observe them (except perhaps with a quantum mechanical tunneling microscope), present a huge obstacle to the light photons. Recall that light, though it is electromagnetic energy, has what is called particle-wave duality, and it can appear to be a particle (with zero mass) as well as a wave. The photon is even more tiny than the atom. Remember atomic structure? Let's look. An atom is a nucleus of some kind with an electron or electrons whizzing around it. It is the electrons that define the "skin" or the "outer boundary" of the atom. The outside perimeter of the electron cloud is the "fence" around the space the atom occupies. And the electrons mean business. The prevent other atoms from "moving into" their space. And the electrons are in motion all the time, sorta like they are on patrol in their orbitals. What happens when a photon shows up? A photon, which is electromagnetic energy, interacts with the electrons in the atoms of the medium through which it is trying to make its way. Each medium treats light differently (and a given medium treats different wavelengths of light differently, too!), and this results in the photons kind of "bouncing there way" through the medium. The speed of the photons, the speed of the light, is constant, but the "delay" introduced by interference caused by the electrons of the atoms and/or molecules of the medium through which it is traveling makes it appear to slow down. The photons have interacted with the electrons of the atoms in the medium and slowed the passage of light through the medium without actually slowing up the light itself. There is a bit more to this, but it isn't practical to conduct a course in quantum electrodynamics here. The long and short of it is that the light interacts with the electrons of the atoms in the medium through which it's moving, being absorbed and re-emitted as it goes.

Answer 2

The refractive index of a medium is its optical density. As it gets denser, light travels slower and vice versa. That's why light travels slower in glass rather than air, because glass is the relatively denser medium (having a higher refractive index than air).