Speed of light is always the same in a medium??

[Lostintime]

I was taught that speed of light was the same for all light in a given medium.

But wiki and my friends disagree, saying different lights can have different speeds.

Who is right??

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[kasho11]

Wiki is right as always 🙂

n = c/v

n is the index of refraction of the medium, c is speed of light in a vacuum - 3x10^8, v is speed of light in that medium. Hence it will always be slower than in a vacuum.


Edit I was responding to your second line, for a given medium for MCAT purposes, yes the speed will always be the same.

 

[milski]

For a light with given wavelength - yes, it's always the same. There is a slight difference between the speeds for different wavelengths in the same media. The phenomenon is called dispersion. For most practical purposes you can ignore that difference and assume that c_f is the same for all f.

 

[EnginrTheFuture]

I was taught that speed of light was the same for all light in a given medium.

But wiki and my friends disagree, saying different lights can have different speeds.

Who is right??

When you say "different light"... if you mean different frequencies of light then you are wrong and your friends are right. Don't admit that to them 😀 but they are right.

If I have a radio wave (large wavelength) and a ray of purple light (400 nm), when they traverse anything but a perfect vacuum, the longer wavelength will travel faster than the shorter. Like someone mentioned above, this is the law of dispersion. Longer wavelengths will have an n closer to 1 whereas the shorter wavelengths will have an n larger than one. This is both the reason why short wavelength light bends more during a change in medium and also why it travels slower in each of those mediums

 

[UrshumMurshum]

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[UrshumMurshum]

in a vacuum the speed of all forms of light is constant. However in a medium there can be dispersion which basically says that the index of refraction increases for higher energy photons.

v = c/n

Therefore blue light travels slower in a medium with n>1 than red light.

 

[Lostintime]

Wait... so what's going on here?

kasho11 and milski say speed remains the same.
UrshumMurshum and EnginrTheFuture say the speeds are different.


Of course, I only care about MCAT boundaries. I have no need for extraneous information.

So.... speed is always the same?

 

[hellocubed]

We had a long topic here a few months ago where we discussed this with Milsiki. Our conclusion was this:


1.) Speed is same for ALL waves ALWAYS

2.) You ONLY ONLY ONLY consider speed to be different when the question asks you about the instant of when a wave crosses a medium. From then on, the higher energy wave will be slower.

 

[milski]

We had a long topic here a few months ago where we discussed this with Milsiki. Our conclusion was this:


1.) Speed is same for ALL waves ALWAYS (ML) in the same media.

2.) You ONLY ONLY ONLY consider speed to be different when the question asks you about the instant of when a wave crosses a medium. From then on, the higher energy wave will be slower.

Kind of, sort of. Dispersion is on the list of topics for the MCAT. You can ignore it in all problems where it is not explicitly mentioned but it needs to be considered when mentioned. It does not apply only to crossing mediums - the speed will be different through the media too. No idea what sort of problems you could get base on that though.