# Refraction of light

1. Jul 21, 2014

### wellorderingp

So I have this very primitive doubt-

When light travels through a medium its wavelength changes,so does the colour even change, as it is also associated with it? I mean that if we had a photographic plate as a detector to detect the colour change in the medium.
Like if I send blue light through a dense medium will the colour change to something else in the medium and could we detect it?

2. Jul 21, 2014

### maajdl

No, the color doesn't change.
The color is determined by the frequency which remains unchanged.

This is logical after all!
Any detector, your eyes, won't measure the wavelength in the medium, but only the frequency of the received light.

3. Jul 21, 2014

### wellorderingp

But wavelength and frequency are associated so wont that mean that even wavelength is one of the characteristic of colour?

4. Jul 21, 2014

### wellorderingp

If I say a certain colour fall in a particular range of wavelength and after refraction the range changes and so will the colour.
Now I agree that when the light will leave the medium it will be in its original wavelength but suppose we had the ability to look inside the medium would we see a different colour?

5. Jul 21, 2014

### Staff: Mentor

Color is subjective and depends on the person (for example, my stepdad is red-green colorblind). Science generally uses the terms wavelength and frequency since both of these are measurable. So no, the color of the light doesn't change since it has no color until perceived by a person. The wavelength changes of course, but the frequency remains the same.

6. Jul 21, 2014

### Staff: Mentor

No, we would not. As maajdi said, the frequency determines the color. When light passes through a medium, its wavelength changes, and then returns to its original value when it leaves the medium, but its frequency remains constant throughout.

7. Jul 21, 2014

### A.T.

The eye is made of a medium, and that's the medium that determines the frequency / wavelength relationship that the light sensitive cells experience. Are you asking, if we would see different colors, if the medium of our eye was replaced with a different one?

8. Jul 21, 2014

### highschoolphys

Yes, wavelength and frequency are related by this equation: v = fλ, where v is the speed of the light in the medium, f is the frequency, and λ is the wavelength. However, frequency is what determines the perceived color (not wavelength).

When light refracts at an interface between two media, its wavelength AND its speed change. Frequency (and hence perceived color) remains the same for any given observer.

Hence, when light enters a different medium, its change in wavelength does not imply a change in frequency. This is due to the fact that the speed of the light also changes upon the light entering the different medium. In fact, the factor by which the wavelength changes is equal to the factor by which the light's speed changes. If you look at your hand under white light in air, it will appear the same color as your hand under white light in water.

Last edited: Jul 21, 2014
9. Jul 21, 2014

### wellorderingp

Is there some kind of device which detects colour or is it just a perception of human vision? IF there was this kind of device and it is placed inside the medium so it would detect colour change,right?

10. Jul 21, 2014

### UltrafastPED

Analyzed via Maxwell's equations it is the frequency that drives the system; only the wavelength changes in order to satisfy the local speed of light in that media.

The easiest measurements are wavelength - as with a prism or a grating; frequency measurements are always more complicated.

For example, if a diffraction grating is etched inside of a glass fiber; this is actually done all of the time if optical fibers: see https://en.wikipedia.org/wiki/Fiber_Bragg_grating).

If the frequency was changing with the variations in the index of refraction then a quantum analysis would require that the energy of the photons would be changing; this is a consequence of Planck's relation Energy=h*frequency. This does not occur in ordinary linear optics.

11. Jul 21, 2014

### sophiecentaur

You could be talking about a TV camera. It analyses the light from a scene and it describes the colour (i.e. what you would experience as a colour)in terms of three values (RG and B), which can be used to give a pretty good matching colour on a TV display. Colour TV 'knows about' the eye / brain's response to the light from a scene and can fool the eye by producing a matching colour in a totally different way from the way the original colour was produced. A standard TV display is capable of resolving 'millions of colours', which isn't bad for a measuring system, is it? And we can do much better if we want to.

Eyes do not see wavelength - they see frequency because they are electrochemical devices that respond to the energy (= frequency) of the photons that arrive.

12. Jul 21, 2014

### highschoolphys

To answer the question AT posed: from a physics standpoint, replacing the medium of the eye (the vitreous humor) would not cause colors to look differently. The reason for this is that the medium of the eye determines the wavelength of the light we see, but our eye's medium does not determine the frequency of the light we see. Only frequency (and not wavelength) determines the perceived color.

Place a green laser pointer in water and record it with an underwater camera. Then place the camera and the green laser pointer in a different medium, like air. Record the green laster pointer in the air and compare to the recording taken under water. The light from the green laser pointer will have a smaller wavelength in the water and the laser beam will be moving slower in the water. In air, the light will have a larger wavelength and the beam will be moving faster. In both air and water, the light will have the exact same frequency. In both air and water, the light will appear the exact same color of green to the camera.

Last edited: Jul 21, 2014
13. Jul 21, 2014

### Staff: Mentor

I'd like to point out that our photoreceptors are themselves a medium that the light has to enter to be detected. So whatever medium comes before the receptors is irrelevant, as the light's wavelength will change upon entering a photoreceptor and being detected.

14. Jul 21, 2014

### highschoolphys

Drakkith, could you elaborate? To what extent does a photoreceptor cell transmit light? I thought light was simply absorbed upon contact with the outermost part of the photoreceptor cell, which produces a difference in membrane voltage. Once the light is absorbed, it ceases to be transmitted. Hence, if light is absorbed by the photoreceptor cell and not transmitted through the cell, then the photoreceptor cell doesn't function as a medium of the light. However, I don't know what part of the photoreceptor cell absorbs the light and whether the light must pass through some initial part of the photoreceptor cell prior to being absorbed by a different part of the cell.

Specifically: what part of the photoreceptor cell does light pass through, and what other part of the photoreceptor cell absorbs the light? Thanks!

15. Jul 21, 2014

### Staff: Mentor

Structurally, a cone cell (the color receptor of the eye) faces backwards, with the color filter/absorbing part of the cell towards the back of the eye rather than the front. This means that light has to pass through the entirety of the cell before being absorbed and detected.

http://en.wikipedia.org/wiki/Cone_cell#Structure

16. Jul 21, 2014

### highschoolphys

COOL, thanks!

17. Jul 21, 2014

### wellorderingp

Thanks!! That was very much satisfactory.
So the colour wont change because it is determined by the frequency.

18. Jul 22, 2014

### DrDu

Implicitly, the thread contains two interesting questions:
1. Is it possible to build a light detector which measures wavelength and not frequency?
2. Are media possible which change the frequency of light?
I think the answer to both questions is affirmative, in principle.
E.g. to satisfy 1, you could bring in some fluorescent ions in the medium in a well defined distance and measure the coherence of the fluorescent light.
In fact a very similar arrangement was used to produce one of the first colour photographs: http://en.wikipedia.org/wiki/Lippmann_plate for which Lippmann got the Nobel prize in 1908.
2. occurs in several processes involving non-linear optics. A more exotic theoretical possibility would be a space- time crystal http://en.wikipedia.org/wiki/Space-time_crystal

19. Jul 22, 2014

### Staff: Mentor

In a Michelson interferometer, moving one of the mirrors through a distance λ/2 changes the path length of that "arm" of the interferometer by λ and causes the interference pattern to shift by one cycle.

20. Jul 22, 2014

### sophiecentaur

A broadband detector (which doesn't care about frequency), in conjunction with any form of interference generating structure - like a diffraction grating (or a prism, of course) will tell you the wavelength of the light reaching it and within the wavelength measuring device. It will tell you nothing about the wavelength of the light on its path to the detector. It could have passed through glass, water, vacuum etc. etc. but would not show anything of this history - just its frequency.
Take a calibrated diffraction grating and operate it under water (quite possible to do). It will still give you a value for the wavelength it measures - but the frequency of the light will not correspond to that wavelength in air.

We seem to have done all the possible combinations in this thread. Surely the main message is that

c = fλ
where c can vary with the medium.
??