MadSci Network: Physics

Re: Does polarization change the color of light?

Date: Thu Nov 10 18:23:31 2005
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 1131525470.Ph

Gosh, you asked a bunch of questions. I think I can answer most of them; let's take them one at a time.

One, does polarization change the color of the light perceived,not just its amount or intensity, since photons emitted in a certain plane are absorbed?

The color of light depends on its wavelength: the longest wavelength we can see is deep red, and the shortest wavelength we can see is purple. One can also say that color depends on frequency, since frequency and wavelength are closely related. Purple light has the highest frequency we can detect with our eyes, and reddish light the lowest frequency.

Polarization does not modify the wavelength (or frequency) of light, and so it does not change the color of a light beam.

However, there are some situations in which the color of light might change as an indirect result of polarization. Let me explain. One way that light may be polarized is when it bounces off very small particles of dust, if the pieces of dust happen to be aligned in a certain way. Now, in some situations, the fraction of an incoming light beam which actually interacts with a dust particle may depend on wavelength; for example, blue light rays may strike dust particles more strongly than red ones. Let's say that 100 blue photons and 100 red photons from a lamp fly into a cloud of dust. It may turn out that 50 of the blue photons run into a dust particle, but only 20 of the red photons collide with a dust particle. That means that half of the blue light may be polarized, while only one-fifth of the red light may be polarized.

If you look at the outgoing light beam with a device which detects only polarized light, then you may conclude that the source of light is blueish -- even though the incoming light beam was equally split between red and blue. The change in color is due to the different probability of light rays of different wavelengths interacting with dust particles ... but it might take a polarizing detector to notice this change.

Also, since the excited electrons of light vibrate in all directions, how can two crossed polarizers block all light

You wrote "excited electrons of light", but I think you meant to write "photons of light", didn't you?

This is a hard question to answer without using some mathematics. It's true that most sources of light, like an ordinary light bulb or the Sun, produce photons which are polarized in all directions equally. That means that the electric fields associated with one photon may point in a North/South direction, the fields associated with a second photon in an East/West direction, the fields associated with a third in NorthEast/SouthWest direction, and so on.

A polarizing filter will interact with the photons which pass through it in a peculiar way: it will, in essence, destroy all the parts of a photon's electric field which is not exactly lined up in a particular direction. Suppose that a filter is aligned North/South. The first photon passes through it unchanged: still North/South. The second photon was pure East/West, with no North or South at all, so it is blocked. The third photon had some North and some South, so it passes through in part: the outgoing version will be North/South only, without the East/West component ... and so it will be weaker than it was going in. Another way to say that is to say that a portion of that incoming light ray is absorbed by the filter (that's the East/West portion) and another portion is passed through it (that's the North/South portion).

and how can polarizers at an angle twist the light? Do the photons "bump" into the long-strand molcules and thereby change direction?

Uh-oh. You've just reached the edge of my understanding of the phenomenon. I do not know the details of this process you mention: the way that some very long, thin molecules can change the direction of polarization of a light beam. I suspect that the mechanism is pretty complicated; after all, it requires a molecule made up of many atoms. I'm afraid that you'll have to consult a good university textbook on optics or materials science to find out. Sorry :-(

However, I do know that once you have such a material, you can use it to create very useful devices. The liquid-crystal display (LCD) is one of these, and appears all over the place: on watches, clocks, VCR control panels, even in some very expensive flat TV and computer screens. You can read a short explanation of the way in which LCD materials employ polarization at

I'm sure you can find more detailed information in the same textbook which explains how long molecules cause the light beam's polarization to twist. Good luck!

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