dan lasota's masters in education portfolio for online innovation and design

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Dialing in the Polarizer

30 March 2015 project

Before I have the flint glass prism in hand, I’ll need to devise a way to have the students determine the amount of rotation taking place in the polarized light.

I’ve been told, that diode lasers already produce a nearly polarized light. (And are monochromatic to boot!). I had planned on purchasing some inexpensive lasers of varying frequencies. There are many options to choose from online, especially those lasers meant as being pointers. The problem is, they are all battery powered. Battery power is not an option in the lab, where the laser needs to be on for long periods of time. I wanted a laser that could be plugged into the wall.

The other factor running against a hand-held pointer type of laser is that they start to overheat, and the intensity of light varies. Again, this makes lab use impossible, especially in a situation where I needed to measure the intensity of light. Why? Well polarization.

If light that is already polarized travels through a polarizing filter at 90 degrees to its polarized angle, nearly all of its light will be attenuated, or blocked. You can see this easily by holding two pairs of polarized sunglasses up to each other and rotating one pair at right angles. The pair of lenses will become black.

Red Laser and Polarizer

Polarizer with Degree Markings

The above two pictures show my lab laser and the polarizing filter. Without any other factors in play, the intensity of the laser light should dip to almost zero when the polarizer is at right angles to the incoming angle of the laser beam. What I did was slowly rotate the polarizer and note where the dips in intensity where. It turns out that they were at 0, and 180 degrees on the polarizer. I decided I needed more precise measurements to get the characteristic curve, so I measured every two degrees:

What measures the intensity of light is a photometer of some sort. It was attached to a generic input device which has a USB interface and feeds data to a program running on one of the lab computers.

photometer

The odd thing about the photometer (shown directly above), is that it was not heavy. It’s spatial positioning is very dependent on the cord attaching it to the computer. I needed something to stabilize it. Luckily for me there was some equipment from another physics lab, in the shape of a metal crown. This device was perfect for holding the photometer steady.

Below are two graphs I made from measuring every two degrees. Basically, the polarizers work best when they are at right angles to the incoming light, but also attenuate to some degree resembling a sine curve with +- 20 degrees. At all other points on the dial, the filters let all the light through. The graphs from both points on the polarizer:

Screen Shot 2015-03-30 at 12.33.48 AM

Screen Shot 2015-03-30 at 12.34.10 AM

This will be the key to knowing how much the incoming laser bear has been rotated. If these dips in the intensity curves have shifted, then the exact angle of rotation can be determined.

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