Wavelength of light changing in a medium

In summary, the speed of light in a vacuum is constant, but the speed of light changes in media of different refractive indices.
  • #1
rpthomps
182
19
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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  • #2
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.
 
  • #3
+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.
 
  • #4
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?
 
  • #5
LURCH said:
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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  • #6
LURCH said:
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.
 
  • #7
rpthomps said:
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
 
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  • #8
Lord Jestocost said:
An elegant presentation of the essential relations can be found in http://www.feynmanlectures.caltech.edu/I_31.html
That is the best way to look at the phenomenon. Unfortunately, people want to introduce Photons into what is essentially a 'bulk' process.
 
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  • #9
rpthomps said:
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.
 
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  • #10
LURCH said:
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. :)
 
  • #11
  • #12
rpthomps said:
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.
 
  • #13
sophiecentaur said:
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.
 
  • #14
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)?
 
  • #15
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.
 
  • #16
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.
 
  • #17
LURCH said:
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.
 
  • #18
Nugatory said:
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.
 
  • #19
DariusP said:
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.
 
  • #20
ZapperZ said:
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.
 
  • #21
This is slightly on topic but a question I have had. Speed is a property of the medium and frequency is a property of the source. Therefore speed decreases, therefore the wavelength decreases (not frequency). The question I have is, does the light change color in the medium, for instance red light in glass will it be green (roughly)? The confusion I have is wavelength is decreased to the blue-green range but the frequency is still at the red range. Is it red or blue-green inside the glass?
 
  • #22
pibcrazy said:
This is slightly on topic but a question I have had. Speed is a property of the medium and frequency is a property of the source. Therefore speed decreases, therefore the wavelength decreases (not frequency). The question I have is, does the light change color in the medium, for instance red light in glass will it be green (roughly)? The confusion I have is wavelength is decreased to the blue-green range but the frequency is still at the red range. Is it red or blue-green inside the glass?
What possible experiment could tell the difference?
 
  • #23
Well, I know it changes the interference pattern. If you shine light underwater and propagates through a slit underwater it changes the interference pattern.
 
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  • #24
pibcrazy said:
Well, I know it changes the interference pattern. If you shine light underwater and propagates through a slit underwater it changes the interference pattern.
Right. That detects a change in wavelength. You were after a change in "color". The challenge is to define what you mean by "color" in this context in a way that can be measured.

If no such measurement can be found then the question vanishes in a puff of philosophy.
 
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  • #25
jbriggs444 said:
What possible experiment could tell the difference?
Refraction shows the difference pretty well. Speed / Wavelength change is the best explanation for refraction.
 
  • #26
pibcrazy said:
The question I have is, does the light change color in the medium, for instance red light in glass will it be green (roughly)?
Wooah there. The Frequency is what determines how your light receptors work. Green stays Green (in case you hadn't noticed when you last looked through a glass of water.)
 
  • #27
sophiecentaur said:
Refraction shows the difference pretty well. Speed / Wavelength change is the best explanation for refraction.
The question was about what "color" light has while is is in a refractive medium. Looking at it after it has emerged does not resolve that question. Though, arguably, since our eyes are filled with a refractive medium, the question answers itself.
 
  • #28
ZapperZ said:
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. ...
Again, pay attention to what is meant by "speed of light" in a medium. This is the group velocity of light ...

Group velocity or phase velocity, as in Wikipedia?
 
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  • #29
The index of refraction of a material is generally a function of the frequency of the light traversing the material; so, it is - as stated in Wikipedia - the ratio of the speed of light in vacuum and the phase velocity of light in a medium.
 
  • #30
jbriggs444 said:
The question was about what "color" light has while is is in a refractive medium. Looking at it after it has emerged does not resolve that question. Though, arguably, since our eyes are filled with a refractive medium, the question answers itself.
Fair enough. But, if you want an example of an easy experiment with radio waves, you can put a probe anywhere you like on the path of a radio waves through a range of media or structures with different values of transmission speed. The frequency never changes (if the probes stationary, of course) but the wavelength can easily be shown to change. What applies to one class of EM wave myst apply to all classes.
The issue of group velocity or phase velocity is not really relevant to this argument; best to stick with the main flow of thought, I think.

But the total sense of the notion that the frequency of waves cannot change can be supported by the question of how the frequency can actually change at an interface.
You have, either to go down the Mathematical analysis of the process or go for a more intuitive approach. The analytical approach demands continuity but intuitive approach involves some 'physica'l thought experiments. Any change in frequency could only be as a result of injection or removal of Energy during the transition. There has to be continuity of displacement along the whole path. Waves with two different frequencies would constantly be 'out of step' at the interface. A Max on the incident wave would need to go to a Zero on the transmitted wave and back again as the two frequencies sweep through each other. Just try to visualise a surface wave on water, going from a deep section (high speed) to a shallow section (lower speed) and then try to describe how those two waves could have different frequencies. What could be the shape of the waves at the interface? Maxes would be piling up from the deep section, unable to get away, via the shallow section.
 
  • #31
DanMP said:
Group velocity or phase velocity, as in Wikipedia?
It's phase velocity.

--
lightarrow
 
  • #32
DanMP said:
Group velocity or phase velocity, as in Wikipedia?

Sorry, it is phase velocity. For some stupid reason, I flipped things around.

Ugh. More coffee!

Zz.
 
  • #33
pibcrazy said:
This is slightly on topic but a question I have had. Speed is a property of the medium and frequency is a property of the source. Therefore speed decreases, therefore the wavelength decreases (not frequency). The question I have is, does the light change color in the medium, for instance red light in glass will it be green (roughly)? The confusion I have is wavelength is decreased to the blue-green range but the frequency is still at the red range. Is it red or blue-green inside the glass?
It depends on how you define "colour" of the light. Up to now textbooks made you believe the answer was trivial :smile:
How do you want to define it? Consider the answers they have already written.

--
lightarrow
 
  • #34
DariusP said:
Please explain the refraction part? I thought light slows down because of absorption and emission.

You are confusing the model with the thing being modeled. Light is the thing we're modeling. One model makes use of the absorption/emission process you speak of, but a full understanding of that involves a deep sojourn into quantum mechanics and quantum electrodynamics.

I attended a series of lectures given by an atomic physicist. This was about 25 years ago and he was talking about building an atom trap. He showed us how to do a lot of atomic physics without any use of relativity or quantum mechanics. Or as he put, there's not a single ##c## or ##h## anywhere in any of these equations.

He wasn't concerned with how light refracts, but the model can be used to successfully explain the phenomenon of refraction of light. This is essentially Feynman's argument, if IIRC. But, anyway, you model the medium as a collection of atomic nuclei surrounded by electron clouds. When an electromagnetic wave passes through this medium, the oscillating electromagnetic field causes those electron clouds to oscillate, and it is this interaction that is responsible for the change in the speed of the electromagnetic wave. Note that the wave is the model, and yes it's a model of light, but the model is not the light. This is where phrases like "light is an electromagnetic wave" can be misleading.
 
  • #35
sophiecentaur said:
Wooah there. The Frequency is what determines how your light receptors work. Green stays Green (in case you hadn't noticed when you last looked through a glass of water.)

And even if you're inside the glass of water.
 
<h2>What is the definition of wavelength of light?</h2><p>The wavelength of light is the distance between two consecutive peaks or troughs in a wave. It is measured in meters (m) or nanometers (nm).</p><h2>How does the wavelength of light change in a medium?</h2><p>The wavelength of light can change when it enters a new medium due to the change in the speed of light in that medium. This change in speed causes the wavelength to either increase or decrease.</p><h2>What is the relationship between wavelength and frequency of light?</h2><p>The wavelength and frequency of light are inversely proportional to each other. This means that as the wavelength increases, the frequency decreases, and vice versa.</p><h2>How does the wavelength of light affect its color?</h2><p>The wavelength of light is directly related to its color. Shorter wavelengths correspond to higher frequencies and are associated with colors such as blue and violet. Longer wavelengths correspond to lower frequencies and are associated with colors such as red and orange.</p><h2>Can the wavelength of light be changed artificially?</h2><p>Yes, the wavelength of light can be changed artificially through processes such as diffraction, refraction, and interference. These processes manipulate the path of light and can alter its wavelength in a controlled manner.</p>

What is the definition of wavelength of light?

The wavelength of light is the distance between two consecutive peaks or troughs in a wave. It is measured in meters (m) or nanometers (nm).

How does the wavelength of light change in a medium?

The wavelength of light can change when it enters a new medium due to the change in the speed of light in that medium. This change in speed causes the wavelength to either increase or decrease.

What is the relationship between wavelength and frequency of light?

The wavelength and frequency of light are inversely proportional to each other. This means that as the wavelength increases, the frequency decreases, and vice versa.

How does the wavelength of light affect its color?

The wavelength of light is directly related to its color. Shorter wavelengths correspond to higher frequencies and are associated with colors such as blue and violet. Longer wavelengths correspond to lower frequencies and are associated with colors such as red and orange.

Can the wavelength of light be changed artificially?

Yes, the wavelength of light can be changed artificially through processes such as diffraction, refraction, and interference. These processes manipulate the path of light and can alter its wavelength in a controlled manner.

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