# B Wavelength of light changing in a medium

#### rpthomps

Good afternoon,

I have read that light changes it's wavelength when it enters a different medium because it's speed changes but then I read that the speed of light doesn't change (it's always c) and it just takes longer. So, it is the "observed" wavelength that changes or some such? Any help is greatly appreciated.

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#### mjc123

c - the speed of light in a vacuum - is constant. The speed of light changes in media of different refractive indices. n = cvac/cmed.
The frequency of the light remains the same, so the wavelength changes when the speed of light changes.

#### sophiecentaur

Gold Member
+1
There are so many wrong statements about the frequency changing. This point should be corrected again and again. No one ever suggests how it could change, they just repeat what they thought they heard.

#### LURCH

I believe the OP is referring to a statement they heard (as I have) that light always moves at c, and that the appearant change in velocity when moving through a medium is actually light traveling at c from one particle to another, then being delayed be being absorbed and then re-emitted by each particle. Is that a correct understanding of how light travels through a medium? Also , is it a correct understanding of your question, rpthomps?

#### Nugatory

Mentor
Is that a correct understanding of how light travels through a medium?
No, the interaction between the oscillating electromagnetic field which is light and the charged particles which make up the atoms of the medium is more complicated than simple absorption and reemission. The simple model is a convenient way of explaining why the speed of light is slower in a medium (I've used it myself) but it breaks down when you want to explain frequency-dependent phenomena such as refraction.

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#### sophiecentaur

Gold Member
light traveling at c from one particle to another,
This is a difficult model to maintain when you you consider that the wavelength of the light passing through a transparent solid is greater than the inter molecular / inter atomic spacing. The idea of a shower of point-like photons finding their way through a region full of anchored particles may be attractive at first but the medium as a whole is what influences the waves passing through. That model actually works.

#### Lord Jestocost

Gold Member
2018 Award
I have read that light changes it's wavelength when it enters a different medium because it's speed changes but then I read that the speed of light doesn't change (it's always c) and it just takes longer. So, it is the "observed" wavelength that changes or some such? Any help is greatly appreciated.
An elegant presentation of the essential relations can be found in http://www.feynmanlectures.caltech.edu/I_31.html

Gold Member

#### ZapperZ

Staff Emeritus
2018 Award
Good afternoon,

I have read that light changes it's wavelength when it enters a different medium because it's speed changes but then I read that the speed of light doesn't change (it's always c) and it just takes longer. So, it is the "observed" wavelength that changes or some such? Any help is greatly appreciated.
Please understand that in such cases, we measure the group velocity of light, i.e. we measure either a pulse of light, or a "disturbance". Any student of physics can tell you that the group velocity of anything can easily have a speed that is less than the component of that pulse. This is the speed that is relevant when we talk about the index of refraction of a material. It is not the "speed of photons" or something of that nature.

Zz.

#### rpthomps

I believe the OP is referring to a statement they heard (as I have) that light always moves at c, and that the appearant change in velocity when moving through a medium is actually light traveling at c from one particle to another, then being delayed be being absorbed and then re-emitted by each particle. Is that a correct understanding of how light travels through a medium? Also , is it a correct understanding of your question, rpthomps?
Yes, that is correct. :)

#### sophiecentaur

Gold Member
I will check this out thanks. Maybe I should get a copy of these lectures and read them.
More entertaining than that - there are YouTube videos of many of those lectures. Unfortunately, that guy leaves you thinking you have grasped everything he has said. Next morning you may not feel the same way but you will get a lot out of them, nonetheless.
Walter Lewin gave a set of lectures to his students which are also very entertaining. YouTube has many examples. This link is a lecture about light in a medium and colour perception. It's always worth doing a search for his name on any Physics topic.

#### rpthomps

More entertaining than that - there are YouTube videos of many of those lectures. Unfortunately, that guy leaves you thinking you have grasped everything he has said. Next morning you may not feel the same way but you will get a lot out of them, nonetheless.
Walter Lewin gave a set of lectures to his students which are also very entertaining. YouTube has many examples. This link is a lecture about light in a medium and colour perception. It's always worth doing a search for his name on any Physics topic.
Thanks. I appreciate that.

#### LURCH

Haven’t looked at any of the videos yet, but I have something of a vested interest in this topic. Would I be correct to surmise that this is exclusively a blue-shift phenomenon? Obviously, there is no medium through which light could travel that would make it accelerate beyond c, but does anyone know of any material that causes light to “bunch up”, and become blue-shifted? My guess is that there is no such medium, but maybe somebody here knows of one?

Also, is this effect usually observed evenly across the entire spectrum, or do some frequencies get effected by a certain media, while others remain unneffected (or less effected)?

#### mjc123

I'm not sure blue shift is an appropriate term, at least if taken literally. Colour is a function of frequency, and that does not change on refraction (as distinct from Doppler shifting).
In most media n varies with frequency, this is called the "dispersion" of the medium.

#### LURCH

Oh yes, wavelength not frequency. So frequency stays the same and wavelength gets shorter, right? And the wavelength is never increased by this process? I mean in normal circumstances, when incoming light is coming from vacuum or atmosphere into some medium.

#### ZapperZ

Staff Emeritus
2018 Award
Oh yes, wavelength not frequency. So frequency stays the same and wavelength gets shorter, right? And the wavelength is never increased by this process? I mean in normal circumstances, when incoming light is coming from vacuum or atmosphere into some medium.
If you're asking if there are instances where a medium causes the group velocity to be faster than c, then the answer is yes. This was done way back in 2000 by the NEC group using an anomalous Cs gas, and which had been discussed a number of times in here.

https://physicsworld.com/a/laser-smashes-light-speed-record/

Zz.

#### DariusP

No, the interaction between the oscillating electromagnetic field which is light and the charged particles which make up the atoms of the medium is more complicated than simple absorption and reemission. The simple model is a convenient way of explaining why the speed of light is slower in a medium (I've used it myself) but it breaks down when you want to explain frequency-dependent phenomena such as refraction.
Please explain the refraction part? I thought light slows down because of absorption and emission.

#### ZapperZ

Staff Emeritus
2018 Award
Think about it like that: refractive index defines how optically dense the material is. If light travels from optically rarer (lower refractive index) to an optically denser (higher refractive index) material then light appears to slow down because it takes light longer to travel the same distance. However, the speed of light doesn't change.
You are confusing the cause-and-effect here. We define the index of refraction by the change in the speed of light, or more specifically, the group velocity of light. This means that we measure the speed of light first, then deduce the index of refraction. It is the optical property of the material that determines the index of refraction, not the index of refraction defining the optical density of the material.

Again, pay attention to what is meant by "speed of light" in a medium. This is the group velocity of light, often measured by using a light pulse. It is not the speed of photons. And simply saying "speed of light" is vague since we have phase velocity, group velocity, etc.

Zz.

#### DariusP

You are confusing the cause-and-effect here. We define the index of refraction by the change in the speed of light, or more specifically, the group velocity of light. This means that we measure the speed of light first, then deduce the index of refraction. It is the optical property of the material that determines the index of refraction, not the index of refraction defining the optical density of the material.

Again, pay attention to what is meant by "speed of light" in a medium. This is the group velocity of light, often measured by using a light pulse. It is not the speed of photons. And simply saying "speed of light" is vague since we have phase velocity, group velocity, etc.

Zz.
Yes, I got a bit confused after reading Nugatory's post and actually deleted my post a second before you replied.

"Wavelength of light changing in a medium"

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