Can the speed of light change?

In summary: However, the speed of light remains the same for all observers. This is because the apparent slowing down of light is due to the interaction of photons with the atoms in the medium, but the speed of the photons themselves does not change. This phenomenon is known as refraction, and it is not a new discovery. It has been widely studied and understood in the field of physics. Additionally, the speed of light can also be affected by gravitational fields, but this does not contradict the fact that the speed of light remains constant for all observers.
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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
 
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  • #2
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.
 
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  • #3
Is light changing speed a relatively new discovery?
 
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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.
 
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kolleamm said:
Is light changing speed a relatively new discovery?
No.
 
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rcgldr said:
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.
 
  • #7
rcgldr said:
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.

phinds said:
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.
 
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So light can change speed? It doesn't always travel at 186,000 mi/s ?
 
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kolleamm said:
So light can change speed? It doesn't always travel at 186,000 mi/s ?
See post #2
 
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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.
 

What is the speed of light?

The speed of light is a physical constant that is defined as approximately 299,792,458 meters per second in a vacuum.

Can the speed of light change?

According to the theory of relativity, the speed of light is a fundamental constant and cannot be changed. However, there are some theories that suggest the speed of light may have been different in the early universe.

What factors affect the speed of light?

The speed of light is affected by the medium it is traveling through. It travels slower in materials such as water or glass compared to a vacuum. It is also affected by gravity; light bends when passing through a strong gravitational field.

Has the speed of light ever been measured to be different?

There have been experiments that have claimed to measure a slight variation in the speed of light, but these have not been widely accepted by the scientific community. The current consensus is that the speed of light is constant and cannot be changed.

Why is the speed of light important in physics?

The speed of light plays a crucial role in many fundamental laws and equations in physics, such as the famous E=mc² equation. It also serves as a constant in the theory of relativity, which has greatly shaped our understanding of the universe.

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