How does light 'speed up'?
 

The speed of light is at it's highest in a vacuum. In earth's atmosphere it travels slightly slower. In certain crystals it slows down a lot. What I don't understand is when it has passed through the substance or passed through the earth's atmosphere, it speeds backup again. Once it is in a vacuum it returns to it's standard (approx. 100,000 Kps) speed. How does it do that?

Light moves slower through transparent substances because it is interacting with particles on the way through, but between those particles it's moving at the speed of light through a vacuum. You get different speeds of light because these interactions reduce the average speed of light as it moves between two points.

It's not like changing mediums slows or speeds light. Rather, changing mediums increases the average number of interactions that light is going to have to deal with on it's way through.

This is my crude understanding of this issue, and I'm sure that somebody else can improve upon or correct my explanation.

its that light is supposed to have the same energy of hf always where h is plank's constant and f is the frequency ot the light. since it has the same energy, everywhere, frequency has to not vary.

so the only thing that gets affected in different mediums is the wavelength, which is something like lamda1(n1) = lamda2(n2) where lamda is wavelength and n is the refractive index of the medium.

This is the result of interference with atoms and molecules in the medium so that it takes more energy to travel a given length in a higher refractive index medium than a lower one.

once you exit the high index medium, the same energy lets you "speed up" again since you're don't have that extra resistance.

sort of like if you're driving and aim to keep the same rpms throughout a long drive, you must slow down going uphill and speed up going down hill, but once you're level again you'll be at the same speed as before if you always try to keep your rpms constant.

of course as far as light is concerned, there is no downhill. there are flats and different degrees of steepness of your uphills

Thank you. One thing wlcm, I would use the analogy of driving along at the same RPMs and hitting a gust of wind. The car slows down, and then returns to normal speed after the wind subsides. Either way the explaination was very clear and helpful.
Bent

its like sound in air and sound in water, the mediam changes the rate

a little off the original question... but an interesting article about light speed. here they discuss how they slowed the speed of light to a pokey 17 meters per second.

http://www.rowland.org/atomcool/lightspeed.pdf

This very observance is why scientist still can't make up their mind about whether or not light is pure energy, or if it has a particle mass (photon).

Light is neither pure energry nor photon (mass) as we know it. That's what makes it so fascinating.

This guy has the right idea here.

Check it out