# Why does the speed of light change in different media?

• ColdheartedGod
In summary, the speed of light is a universal constant in vacuum and has wave-particle duality. However, it slows down in different media due to interactions with particles, resulting in a decrease in wavelength but not frequency. This can be observed in both macroscopic and microscopic points of view. While it may seem counterintuitive, the speed of light in a medium is not influenced by the measurement method, but rather by the properties of the medium itself. The concept of "vacuum fluctuations" further complicates the precise measurement of the speed of light, but it remains a fundamental constant in our universe.
ColdheartedGod
In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down?
Macroscopically speaking, the speed of light does slow down. What about in the microscopic points of view？If as particles, photons are much smaller than electrons and nuclei, so basically it's moving in vacuum with frequent bouncing. Can we say we measure the speed in other media slower than that in vacuum is because photons don't travel alone a straight line as in vacuum but twisty lines due to the bouncing with other particles?
My teacher taught me, if we treat light as waves, when the light enter a new optical dense media, the light rays will collide with particles, lose energy and slows down. But it doesn't make sense when they reenter a media with smaller refractive index, the light rays speed up.
Personally, I cannot understand why a universal constant which plays such a huge role in relativity would be influenced easily by media. With the vacuum fluctuation, we cannot have a real "vacuum", thus the speed cannot be precisely measured. There must be something special about it which make up the speed limit of our universe！！！

ColdheartedGod said:
My teacher taught me, if we treat light as waves, when the light enter a new optical dense media, the light rays will collide with particles, lose energy and slows down.
This was wrong in many ways. The photons that are not absorbed by the medium (some of them will be) do not have their Energy (=frequency) changed. The power of the light beam will be attenuated; the interaction of the unattenuated waves and the substance introduces a lag, which means there is a delay on the way through. That means that the speed of the wave will be measured as slower. It is the wavelength but not the frequency that changes. Once the ray emerges into vacuum (or near vacuum) the wavelength goes back up so that fλ is back to c. (No acceleration is involved - it's not Mechanics)
ColdheartedGod said:
With the vacuum fluctuation, we cannot have a real "vacuum", thus the speed cannot be precisely measured. There must be something special about it which make up the speed limit of our universe！！！
From your first quote to to this quote requires a massive jump in understanding. "Vacuum Fluctuations" are a very sophisticated concept.
c is so fundamental that you can't really hang it on the measurement method; it's more the other way round. There's more to c than just a speed limit. Wherever you are, you will measure c as the same. (There's no other speed like that; all other speeds depend on the observer's situation)

ColdheartedGod said:
In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down?
Macroscopically speaking, the speed of light does slow down. What about in the microscopic points of view？If as particles, photons are much smaller than electrons and nuclei, so basically it's moving in vacuum with frequent bouncing. Can we say we measure the speed in other media slower than that in vacuum is because photons don't travel alone a straight line as in vacuum but twisty lines due to the bouncing with other particles?
My teacher taught me, if we treat light as waves, when the light enter a new optical dense media, the light rays will collide with particles, lose energy and slows down. But it doesn't make sense when they reenter a media with smaller refractive index, the light rays speed up.
Personally, I cannot understand why a universal constant which plays such a huge role in relativity would be influenced easily by media. With the vacuum fluctuation, we cannot have a real "vacuum", thus the speed cannot be precisely measured. There must be something special about it which make up the speed limit of our universe！！！

This question was asked recently. See:

https://www.physicsforums.com/threa...ight-smaller-in-a-medium.977743/#post-6236852

sophiecentaur
ColdheartedGod said:
Personally, I cannot understand why a universal constant which plays such a huge role in relativity would be influenced easily by media.

Note that this is entirely the wrong way to think about physics. This is almost like a political argument! Light is electromagnetic radiation and a vacuum is a vacuum. Hence the speed of light in vacuum is constant. A medium, however, has charged particles that interact with EM radiation. There is no reason, therefore, to presume that the speed of light in a medium is the same as the speed of light in a vacuum.

Note, however, that the speed of light in vacuum is also "invariant", meaning the same in all inertial reference frames. That is the key property that underpins relativity and disrupts the notions of classical physics.

sophiecentaur

## 1. Why does the speed of light change when it passes through different materials?

The speed of light changes when it passes through different materials due to the interaction between light and the particles of the material. Light travels through empty space at a constant speed, but when it enters a material, it interacts with the particles of that material, causing it to slow down.

## 2. How does the density of a material affect the speed of light?

The density of a material plays a significant role in determining the speed of light. In general, the denser the material, the slower the speed of light will be. This is because the particles in a denser material are more closely packed, making it more difficult for light to pass through without interacting with them.

## 3. Does the color of light affect its speed in different media?

No, the color of light does not affect its speed in different media. The speed of light is determined by the properties of the material it is passing through, not the color of the light itself. However, different colors of light may interact with different particles in a material, resulting in varying degrees of refraction.

## 4. Why does light travel faster in air than in water?

Light travels faster in air than in water because air has a lower refractive index compared to water. This means that light can pass through air with less obstruction from the particles, resulting in a faster speed. In contrast, water has a higher refractive index, causing light to slow down as it interacts more with the particles in the water.

## 5. Can the speed of light be changed in a vacuum?

No, the speed of light cannot be changed in a vacuum. In a vacuum, there are no particles for light to interact with, so it travels at its maximum speed of approximately 299,792,458 meters per second. This is known as the speed of light in a vacuum and is considered a fundamental constant in physics.

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