how does light slow down?

Question

I've known that light always travels at 186,000 mps "in a vacuum." How is it then that light slows down when it comes in contact with matter?

Answer 1

The photons interact with the matter as the pass through it. This makes the effective speed of the light slower. The type of interactions are described by quantum electrodynamics.



The older, wave model of light describes light as being formed of complementary magnetic and electric field waves. Different substances have different permabilties to the electric and magnetic field, and so slow down the passage of light. This theory is good, but the quantum theory is better as it explains why matter has different permambilites to magnetic fields.

Answer 2

Light slows down depending on the medium it is traveling through. For example, nuclear reactors all produce an eerie blue glow. That is because they are submerged underwater (for cooling), and when particles are given off from the nuclei, they are actually moving faster than the speed of light IN WATER, which causes the blue glow called Cherenkov radiation.

Answer 3

It doesn't. The photons get absorbed and re-emitted. Thus, they seem to start and stop. The speed between atoms is still c. It is like cars driving through a city with stoplights. The cars stop at every light and go the speed limit between stoplights. So, the AVERAGE speed is less than the speed limit