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.

 

[mjc123]

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

 

[sophiecentaur]

+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]

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.

 

[sophiecentaur]

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]

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

 

[sophiecentaur]

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.

 

[ZapperZ]

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. :)

 

[rpthomps]

An elegant presentation of the essential relations can be found in http://www.feynmanlectures.caltech.edu/I_31.html

I will check this out thanks. Maybe I should get a copy of these lectures and read them.

 

[sophiecentaur]

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]

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]

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.

 

[pibcrazy]

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?

 

[jbriggs444]

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?

 

[pibcrazy]

Well, I know it changes the interference pattern. If you shine light underwater and propagates through a slit underwater it changes the interference pattern.

 

[jbriggs444]

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.

 

[sophiecentaur]

What possible experiment could tell the difference?

Refraction shows the difference pretty well. Speed / Wavelength change is the best explanation for refraction.

 

[sophiecentaur]

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.)

 

[jbriggs444]

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.

 

[DanMP]

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?

 

[Lord Jestocost]

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.

 

[sophiecentaur]

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.

 

[lightarrow]

Group velocity or phase velocity, as in Wikipedia?

It's phase velocity.

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lightarrow

 

[ZapperZ]

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.

 

[lightarrow]

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
How do you want to define it? Consider the answers they have already written.

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lightarrow

 

[Herman Trivilino]

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.

 

[Herman Trivilino]

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.

 

[Herman Trivilino]

It depends on how you define "colour" of the light. Up to now textbooks made you believe the answer was trivial

I still believe it to be trivial. Show a kid some different colored objects and tell him that people name this particular color, say, green. Then, when you see that same color somewhere else and at some other time, call it green.

If you want to go deeper find out what's happening inside the kid's body when he sees the color he calls green. That part isn't trivial, but assigning names is trivial.

 

[sophiecentaur]

Assigning wavelengths to colours is very naughty.

 

[Nugatory]

I still believe it to be trivial. Show a kid some different colored objects and tell him that people name this particular color, say, green. Then, when you see that same color somewhere else and at some other time, call it green.

I cannot resist pointing out that I know someone who sees limes and carrots as the same color... wavelengths and frequencies are objective physical phenomena, colors much less comfortably so.

 

[sophiecentaur]

I cannot resist pointing out that I know someone who sees limes and carrots as the same color... wavelengths and frequencies are objective physical phenomena, colors much less comfortably so.

Colours are a cultural thing that are a common descriptive language that works for most people. The majority of people of one culture will agree, roughly, about the name to give the colour of an object although they may not choose exactly the same paint from a shade card to 'match' that object. The success of modern colour TV systems demonstrates this.

 

[lightarrow]

I still believe it to be trivial. Show a kid some different colored objects and tell him that people name this particular color, say, green. Then, when you see that same color somewhere else and at some other time, call it green.

If you want to go deeper find out what's happening inside the kid's body when he sees the color he calls green. That part isn't trivial, but assigning names is trivial.

If you, then, define "colour" as the name we give to a specific perception, in addition to what sophiecentaur and Nugatory wtote, we have to say that colour perception is an incredibly complex phenomenon: sometimes you can see coloured spots even on gray surfaces for some instants (positive and negative post-image) or even see an entire complex image variously coloured where there is just a two-colours image, you can constantly see coloured shadows where colours cannot exist at all (coloured shadows phenomenon) or see an object as coloured in red under artificial light and green under the Sun (Alexandrite stone), and a lot of other things. Buy a good book on light and colours and you'll discover an entire new world...

Anyway, that was only the "perception" part. The OP could define "colour" in a different way, more technical, even because this must be the case, if we want, e. g. send the right and precise bits to a printer or to a screen to reproduce the right colours on a picture or on the display.
A more precise question he could ask could be:
a) would a picture change if a film exposed to light were in glass (e. g. ) instead of air? Here we would understand better if in this case the process depends on the light frequency (which is the same, with air or glass) or on the light wavelength, which is different (it's the first);
b) if I used a monochromatic red light source, how would the diffraction pattern in the Young experiment change if all the apparatus would be immersed in, let's say, carbon tetrachloride, instead of air?
And so on.

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lightarrow

 

[DanielMB]

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.

I think it is not a convenient way of explaining why the speed of light in a refractive medium is slower than in vacuum, if it were certain (absortion-re-emition delay) speed would depend on the width of the refractive material that light should cross, but it is not, so it is deeply wrong

 

[DanielMB]

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?

No, color is about perception, when your eye receive an image, an opsin molecule absorves a photon and transmits a signal to the photoreceptor cells, photon absortion is about frequency, frequency is the stimulus that matters at the first stage in color perception (not wavelenght), but after that color perception is a complex world ...and as it was said in this thread frequency is determined by the source (in this case we can suppose light reflected from an object)

 

[sophiecentaur]

a) would a picture change if a film exposed to light were in glass (e. g. ) instead of air?

Good question. If it happened, they would need different photo sensors, depending in the substance they're embedded in. (I'm assuming that the effect would have to be as great as the dispersion which we can measure. Another consequence would be that emission and absorption lines would be offset if the frequency changed between an emitter in one medium and an absorption in another - you would get no absorption of monochromatic light.

how would the diffraction pattern in the Young experiment change if all the apparatus would be immersed in, let's say, carbon tetrachloride, instead of air

It would have to change according to the wavelength change. I can't think how it could be otherwise.

 

[lightarrow]

a) would a picture change if a film exposed to light were in glass (e. g. ) instead of air?

Good question. If it happened, they would need different photo sensors, depending in the substance they're embedded in. (I'm assuming that the effect would have to be as great as the dispersion which we can measure. Another consequence would be that emission and absorption lines would be offset if the frequency changed between an emitter in one medium and an absorption in another - you would get no absorption of monochromatic light.

For conventional pictures it's light frequency which counts, but what about Lippmann's photography?
https://en.m.wikipedia.org/wiki/Lippmann_plate
Here the image is formed because of the interference between incident an reflected light in the emulsion; what happens if the emulsion's refractive index changes appreciably?

how would the diffraction pattern in the Young experiment change if all the apparatus would be immersed in, let's say, carbon tetrachloride, instead of air?

It would have to change according to the wavelength change. I can't think how it could be otherwise.

Of course

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lightarrow

 

[Herman Trivilino]

Anyway, that was only the "perception" part. The OP could define "colour" in a different way, more technical, even because this must be the case, if we want, e. g. send the right and precise bits to a printer or to a screen to reproduce the right colours on a picture or on the display.

All of that can be, and probably is, determined empirically. In other words, a more technical definition of color is not required to accomplish this task.

A more precise question he could ask could be:
a) would a picture change if a film exposed to light were in glass (e. g. ) instead of air? Here we would understand better if in this case the process depends on the light frequency (which is the same, with air or glass) or on the light wavelength, which is different (it's the first);

You mean, like, sandwich a piece of film in between two glass plates? Or even embed a CCD in glass. Again, looking at the colors produced in such a manner is trivial. It requires no more precision than looking at a painting on a wall and identifying the colors seen there.

b) if I used a monochromatic red light source, how would the diffraction pattern in the Young experiment change if all the apparatus would be immersed in, let's say, carbon tetrachloride, instead of air?

That's a question about the measurement of physical properties such as wavelength, not about a determination of color.

 

[DanielMB]

Color is a hard concept, you can have two color samples with different spectra (!) (suppose mixing different pigments to get a defined color) that under a D5 Illuminant could offer a perfect match, but when you change the illuminant to C you get an imperfect match between the samples (metamerism). Frequencies are not related in a direct way to perceived colors. To define perceived color from an object seeing by an observer, the observer could have use (e.g) a Munsell Chart in his hand under Illuminant C, doing the match, and obtaining Hue-Value-Chroma coordinates, then he could say "this is the colour I've seen in the scene". When you think in perceived colors you should forget the term "frequency", just because the distant stimulus is the Radiant Spectra, if you are looking at a lemon, the green-yellow color you perceived is formed (the distant stimulus) by the contribution (amplitude) of every frequency in the passband filter that our Human Vision System can manage, and this is only the zero-stage in color perception ...

 

[sophiecentaur]

+1

 

[lightarrow]

All of that can be, and probably is, determined empirically. In other words, a more technical definition of color is not required to accomplish this task.
You mean, like, sandwich a piece of film in between two glass plates? Or even embed a CCD in glass. Again, looking at the colors produced in such a manner is trivial. It requires no more precision than looking at a painting on a wall and identifying the colors seen there.
That's a question about the measurement of physical properties such as wavelength, not about a determination of color.

The colours you see on a Lippmann's photograph depends on light wavelengths or on its frequency? This is just an example to explain the reason I asked the OP to refine his question and the reason I wrote...what I wrote.

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lightarrow

 

[sophiecentaur]

The colours you see on a Lippmann's photograph depends on light wavelengths or on its frequency?

What would the colours of light produced by a diffraction grating look like if the grating were under water? (Or for an oil film between glass and water etc. etc.)

 

[DanielMB]

The colours you see on a Lippmann's photograph depends on light wavelengths or on its frequency? This is just an example to explain the reason I asked the OP to refine his question and the reason I wrote...what I wrote.

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lightarrow

Quoting from Wikipedia "The colour image can only be viewed in the reflection of a diffuse light source from the plate" this is the object that you are looking at (a Lipmann's photograph) : Diffuse light reflected from the plate, the radiant spectra reflected for each pixel will contain lots of frequencies, each of them contributing with its amplitude to the spectra you receive in your retina when you focus the just small region allowed by the angle (about 2º) of your fovea (fovea vision as opposite to peripheral vision), what it is happening inside the film doesn't care, you are receiving light reflected from the plate. The perturbation that means in the electromagnetic field that constitutes the incoming light is the set of frequencies conforming that spectra, imagine each frequency as a metronome the tic-tac is the perturbation, it doesn't matter if the film is under colored water or you as an observer are under colored water, you will perceive that perturbation independent of the media.
Obviously there are other kind of phenomena that depends on wavelength (standing waves, diffraction, etc.) but when you look at an opaque object in a scene under a defined illuminant, you will receive the reflected light coming from that object discarding the contribution of all frequencies lesser than 430 THz (infrared) and greater than 770 THz (ultraviolet) i.e. [430 THz, 770 THz] after that becomes color perception ...
I have supposed an opaque sample that reflects incoming light, but the spectra could be a refracted/reflected/transmitted (e.g. through a film), but when it leaves the sample is just a pack of information called spectra traveling directly to your retina