# Some light properties

1. Jun 6, 2012

### CAF123

Hi,
I am reviewing some material on refraction/Huygens' principle and I would like some help with a couple of technicalities.
1) Why does light slow down when it enters a denser material? (i.e details on scattering/ microscopic effects )Are the resultant wavefronts closer together?
2) Does it make sense to talk of the acceleration or deceleration of light? Does light decelerate when it enters such a denser material?

2. Jun 6, 2012

### ama

light is refracted when it enters another medium. this is based on the index of refraction of the medium the light enters. Denser mediums have higher indexes, which leads to lower speeds of light. Although acceleration or deceleration usually apply to matter, in this case it would make sense to talk about acceleration because the velocity changes and thats what acceleration is.

3. Jun 7, 2012

### CAF123

But why does a denser medium lead to a decrease in light speed? I am interested in things that happen on a microscopic level - ie ideas like the light has to 'fight' through closer arrangements of molecules etc

4. Jun 7, 2012

### ZapperZ

Staff Emeritus
Please start by reading the FAQ subforum in the General Physics forum.

Also, there is no "acceleration and deceleration" for light. ama is forgetting about the difference between group velocity and phase velocity in a medium.

Zz.

5. Jun 7, 2012

### Khashishi

Light is an oscillating electric field coupled with an oscillating magnetic field. The speed of light comes out of Maxwell's equations. The Maxwell's equations in the absence of free charges gives the wave equation:
$\left(\nabla^2 - { \mu\epsilon } {\partial^2 \over \partial t^2} \right) \mathbf{E}\ \ = \ \ \mathbf{0}$
$\left(\nabla^2 - { \mu\epsilon } {\partial^2 \over \partial t^2} \right) \mathbf{B}\ \ = \ \ \mathbf{0}$
you see the speed of propagation is
$c = {1 \over \sqrt {\mu\epsilon} }$
epsilon is the permittivity of the medium, which is how much the medium is polarized by the electric field. When the medium is polarized by the field, this reduces the field, so high permittivity will have a small E field inside. Now, a changing magnetic field in the "denser" medium generates a smaller E field, so the permittivity acts like a sort of "mass" in the wave, which slows it down.