i was confused on this subject, and I have little knowledge so maybe u guyz culd help.

 

the speed of light, c, is the speed of light in a vacuum correct?

so it is slower on earth because of air molecules and would be even slower going through, for example, water or a prism

 

so is the speed of light isnt constant?

maybe I'm looking at this wrong, culd u guyz explain it

The speed of light in a vacuum is constant.

 

In air it is slightly slower than in a vacuum but as air isn't very dense there is not much change.

 

Place a penny in a glass of water and look at what possition it is in, try pushing your finger streight into the water to touch it and you will probably miss as the light is "bent" where there is a speed difference.

 

Or have you ever done the experiment where a shaft of light is shined into a light block and it is bent, this is due to the light moving slower in the glass.

ok thanks ic now

What happens is light waves (or particles) always travel at c however when it bumps into an atom it is absorbed and then (sometimes) remitted. This process takes fractions of a millisecond (or less!) however you must account for the massive number of atoms in a piece of glass then the light keeps being absorbed and remitted.

 

So what happens is when the light is travelling in the vacum between atoms it travels at c, however the (in glass) almost continual absorption and remission that is happening slows down the light.