B Can the speed of light change?

1. Apr 20, 2016

kolleamm

According to my physics book light changes speed in different mediums such as water. Doesn't the speed of light have to remain constant?

2. Apr 20, 2016

Staff: Mentor

The speed of light in vacuum is a constant and the same for all observers. Light can slow down in a medium. In fact, inside a dielectric medium, you can have things moving faster than light; see Cherenkov radiation.

3. Apr 20, 2016

kolleamm

Is light changing speed a relatively new discovery?

4. Apr 20, 2016

rcgldr

I had the impression that In the case of light going through a medium, the apparent speed slows down due to the rate that electrons absorb and emit photons, but that the speed of the photons as they travel unimpeded remains the same.

On the other hand, to an observer outside of the effect of a particular gravity field, the speed of light would be slower where the gravitational intensity is greater.

5. Apr 20, 2016

phinds

No.

6. Apr 20, 2016

phinds

That is a widely held, but incorrect, belief. There is a recent thread on this. I suggest a forum search.

7. Apr 20, 2016

rcgldr

Thanks. At least I know why I had that impression (might have been related to propagation inside of a laser). I wondered what percentage of light is actually captured and re-emitted while traveling through a medium such as glass.

In the case of a mirror, then almost all of the reflected light is due to interaction with the reflecting surface (some of the light is reflected by the glass). I'm wondering if there is a delay (maybe just a phase shift) of the reflected light.

8. Apr 20, 2016

kolleamm

So light can change speed? It doesn't always travel at 186,000 mi/s ?

9. Apr 20, 2016

phinds

See post #2

10. Apr 21, 2016

Staff: Mentor

11. Apr 23, 2016

atom jana

Light travels in vacuum with a constant speed which is equal to 299, 792, 458 m/s. It slows down when it travels through an optically denser medium such as glass, water, etc.