# Wavelength of light changing in a medium

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sophiecentaur
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2020 Award
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
Homework Helper
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.

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
Lord Jestocost
Gold Member
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
Gold Member
2020 Award
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.

Group velocity or phase velocity, as in Wikipedia?
It's phase velocity.

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lightarrow

ZapperZ
Staff Emeritus
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.

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

Mister T
Gold Member
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.

Mister T
Gold Member
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.

Mister T
Gold Member
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
Gold Member
2020 Award
Assigning wavelengths to colours is very naughty.

gneill
Nugatory
Mentor
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
sophiecentaur
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2020 Award
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.

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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sophiecentaur
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

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
Gold Member
2020 Award
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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Mister T
Gold Member
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.

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 ...

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sophiecentaur
Gold Member
2020 Award
+1

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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sophiecentaur
Gold Member
2020 Award
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.)

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 travelling directly to your retina

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