Can the speed of light change?

[kolleamm]

Main Question or Discussion Point

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

Thanks in advance

 

[DrClaude]

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.

 

[kolleamm]

Is light changing speed a relatively new discovery?

 

[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.

 

[phinds]

Is light changing speed a relatively new discovery?

No.

 

[phinds]

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.

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

 

[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.

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

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.

 

[kolleamm]

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

 

[phinds]

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

See post #2

 

[DrClaude]

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

https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/

 

[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.