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Demonstrating Polarized Interference

Roberta Tevlin, OAPT Editor, Teacher Danforth CTI
Rolly Meisel, OAPT Photographer

The interference of light is a very important concept in senior high school physics and has been for a long time. The polarization of light used to be a minor topic but has become more and more important over the last couple of decades because of its use in LCD screens and 3-D movies and because it is possible to get a cheap class set of polarized filters. This article describes a demonstration that combines polarization and interference.

The image below shows an interference pattern. (Note: All these pictures in this article were taken by Rolly Meisel at last year’s OAPT conference. Thanks, Rolly!)

Is it a single-slit pattern or a double-slit pattern? The overall pattern shows a central maximum that is twice as wide as the other maxima — so this is a single slit pattern. However, within the single-slit pattern you can see almost a dozen double-slit fringes. This combined pattern will show up with any double slit where the slits widths are not ‘infinitely’ small.

just pattern cropped Rolly_edited

OK, but why do we only see the double fringes in the central? Why don’t they appear on the maxima on either side as they usually do? The reason, involves polarization. The image below shows the same set-up, but with the lights on. You can see that a polarizing filter has been placed over the central maximum.

pattern lights up cropped Rolly

If this polarizing filter is removed, the double slit fringes will completely disappear. They will also disappear if the filter is rotated by 45o. However, if it is rotated by 90o, you see the interference pattern return. What’s going on? What you need to know is that there are two other polarizing filters involved in this demonstration. A vertically polarized filter has been placed over one slit and a horizontal one covers the other one.

For this article, I wanted to grab your attention, so I showed you the result before discussing the set-up. However, when I use this demonstration with my students, I describe the set-up and then have them predict what they will see and why. There is usually a wide range of opinions at the start, but after some heated discussion they convince each other that the vertically polarized light of one slit cannot cancel the horizontally polarized light of the other slit. That means that there will be no double slit interference. There will just be a single slit pattern overlapped with another single slit pattern that is shifted by a fraction of a millimetre.

Next, I ask them what will happen if I put a third polarizer between the slits and the screen. Again, there is a lot of discussion until someone asks me which way it will be oriented. At that point, most groups are able to recognize that if the polarizer is vertical or horizontal, it will block the light from one slit and only let the other one through. We will see a dimmer but otherwise unchanged single-slit pattern. However, at 45o, each slit will contribute light at this one polarization and now these two sources can interfere and produce a double-slit pattern.

After making these clear predictions and explaining the reasons behind them, I find that my students (and me) are still thrilled by the demonstration. It still looks a bit like magic.

Details of Demonstration Set-Up
I will be the first to admit, that this is a finicky demonstration. However, it is worth the fuss.

The Laser: You will need a good 3A laser pointer. It can’t be a cheap dollar store one because it is not sufficiently well-collimated. It also can’t be an expensive Metrologic laser because it is elliptically polarized. Laser pointers are partially linearly polarized. You want half of the light to go through each slit, so rotate the pointer so that its polarization is at 45° to vertical. The laser should be held in an adjustable clamp because the positioning must be very precise. You can see the laser pointer, retort stand and test tube clamp in the photo below. 

set up cropped Rolly_edited

The Double-Slits: It is possible to do this demo with a straight pin and two filters, but it is much clearer and dependable with a good double-slit. I use a Cornel plate of slits held up with large binder clips. When you place the small polarizing filters over each slit, check that they are aligned properly. When you rotate a third filter in front of the slits, one slit should disappear and after 90°, it should reappear and the other one disappear. Tape the filters firmly into place and don’t let anyone touch them!

The Room: The bigger and darker the room — the better. The increased distance increases the separation between the fringes and the darkness is needed because each filter cuts the intensity in half. A room of 15 m is nice, but 10 m will do. If you have any trouble with the demonstration, drop me a line and I promise to help trouble-shoot.

Sources of Materials
Double Source: I use the second largest separation on the Cornell slit film (d = 0.70 mm). If you don`t have one of these - buy one. They are fabulous! They have a variety of slit widths, slit separations and number of slits. They come in a package of 10 for $255.99 or as a single one with some other stuff for $115.06. Shipping in Ontario will be around$10. https://sargentwelch.ca/store/catalog/product.jsp?catalog_number=WL3800

Polarizing Filters: You don’t need a class set of filters for this demonstration – but you should have a class set for the learning that comes before it. There are two ways these can be mounted; either on cardboard slides (remember slide projectors?) or in old-fashioned 3-D glasses – not the new plastic ones.
  • 2 cm × 3 cm Slides from Arbour Scientific: If you buy two sets of 50 films (~2 cm by 3cm. mounted on slide holders) it will cost $70 plus $13 for shipping to Ontario.
  • 3-D Glasses from Rainbow Symphony: I prefer these to the slides. First of all, they look funny. Secondly, they are needed when exploring the difference between these glasses and the new 3-D glasses that are circularly polarized. Finally, it makes it easier to explore what happens with three filters in a row. Each hand can hold a half of a pair of glasses cut in half and the head (with one eye closed) can hold the third one. If you buy 100 glasses it will cost $55.00 plus $31.60 for shipping to Ontario.

Related Articles
If you want to look at how polarization is dealt with in quantum physics, you might want to look at my article Quantum Polarization.

If you are interested how the circular polarization of 3-D movies works, you might want to look at my article Polarization of Light and 3-D Movies.
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