Physics 204, Spring 2011

Polarization

Some polarization effects can be seen just by holding up a polaroid sheet and looking through it. Other effects are best seen by putting a sample between crossed polarizers. These two methods don’t exhaust all logical possibilities – feel free to play and invent!

I. Looking Through a Polaroid Sheet:

If you look through a polaroid sheet, especially if you turn the sheet, you are testing the light coming through it for linear polarization. If you find a difference in intensity between one orientation of the sheet and another, the light is at least partially polarized. Try looking around at:

Glare from surfaces

The sky

Laser light scattered out of the beam from milky particles in suspension (DO NOT LOOK INTO THE LASER, I.E. ALONG THE BEAM!)

Ceiling lights

Images viewed through a calcite crystal

Anything else handy

II. Samples Between Crossed Polaroid Sheets (Polarizer and Analyzer)

If you put a sample between crossed polarizers, you are testing the sample to see how, if at all, it changes linearly polarized light into light polarized with a component along the other (perpendicular) direction. The first Polaroid sheet, called the polarizer, prepares linearly polarized light to go through the specimen. The second Polaroid sheet, at 90 degrees to the first, is called the analyzer. It selects the perpendicular direction of polarization in the outgoing light. Try looking at

Geological specimens

A half-wave plate

Clear sugar syrup

Stressed Lucite or other plastic

Stretched Saran Wrap

Glass slides

Anything else transparent

You will see a lot of peculiar phenomena, some of them quite beautiful!

If a polarization effect depends on wavelength, you see colored effects: some wavelengths are better able to pass through the analyzer, the second Polaroid sheet, than others.

For example, a half-wave plate is only a half-wave plate for one particular wavelength. For that wavelength, and for that wavelength only, it “reflects” the polarization direction of linearly polarized light about one of its own built-in axes. Our half-wave plates work for green light, around 500 nm. We have green filters that you can use to select just this wavelength. In green light you will see the simplest version of what the plate does.

If you align the built-in axes of the half-wave plate with the polarizer, the half-wave plate does nothing, and the light coming through it is extinguished by the analyzer. But if you put the half-wave plate axes at 45 degrees to the polarizer, the light coming through it is transformed into the analyzer direction, and all of it comes through the analyzer.

We also have quarter-wave plates. Their effect is even more surprising. If you put a quarter-wave plate’s axes at 45 degrees to the polarizer, it produces circularly polarized light. The analyzer will not be able to extinguish it, showing that it is no longer linearly polarized. Two quarter-wave plates on top of each other (with their fast axes aligned) make a half-wave plate, and as noted above this produces linearly polarized light, but polarized in a new direction. Again, the effect depends on wavelength, so it is best seen using a green filter.

Sugar syrup rotates the direction of polarization by an amount that depends on wavelength and thickness of the sample. That means the light coming through the analyzer will have an intensity that is greater for some wavelengths than others,

i.e., it will look colored. If you use a filter to select just a narrow band of wavelengths,

you will see the extinction of the light when the analyzer is turned the appropriate amount (not necessarily 90 degrees from the polarizer now). The amount of turning of the direction of polarization is proportional to the thickness of the syrup layer, and it depends on wavelength.

Describe in writing the things that you see, and interpret them using the ideas of polarization.