Polarization

Two polarized disks were put into a grooved holder with their polarization axes lined up, and a bright red light emitting diode (LED) was placed on one side, and a light sensor was placed on the other side. Light intensity was measured as a function of the plastic rotating polarizer angle from 0 to 180 degrees. Science Workshop was used to measure the intensity for every 5-degree rotation (Fig 1).

The mineral calcite exhibits birefringence (double refraction), and therefore has two different values for its index of refraction. A calcite crystal was placed on top of print and rotated in order to see the difference between the two rays, the ordinary ray and the extraordinary ray. A polarizer was then used to look at the images formed by the calcite.

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In order to determine the stress points in a plastic object, we used a photoelastic effect where we looked at the object through a polarizer. When the plastic was squeezed together, changes in the color occurred, displaying points with greater stress.

Polarization by reflection from a non-metallic surface was used in this method in order to help distinguish the plane of incidence, and Brewster’s angle. A cardboard disk with a picture on it and a polarizer were placed in the holder, with a piece of black plastic (n=1.491) in between them. Brewster’s angle was determined from tan= n, and=56. We then looked through the polarizer at this angle and were able to determine the polarization direction. We also viewed the reflection of a ceiling light directly above the black plastic in order to determine if it was polarized in this case.

In order to determine polarization by scattering, a jar was filled with water and dried milk, and then a flashlight was shined directly down into the top of the jar. From the side of the jar, we looked through a polarizer at the scattered light. From this we determined if the light was horizontally polarized.

A halogen bulb light source was set up at the end of a yardstick, and a light sensor was set up on the yardstick beginning at 0.1m from the light source. Science Workshop was again used to measure the intensity of light ranging from 0.1m to 1.00m (Fig. 2).

In part one, method one, the curve for my values was greater than that for the calculated values, but was quite similar other than that. This could be due to the fact that our methods weren’t as precise as they could have been and if we had been more precise, our values would have been too.

In method two, the calcite crystal created a double image, and as you rotated the crystal, one image stood still (formed by the ordinary ray), while the other one rotated around it (formed by the extraordinary ray). When the print is viewed through the crystal and then through a polarizer which is being rotated, we were able to see the in-phase and out-of-phase of the polarized light. The images would switch so that only one of them was viewed at a time. This told us that the relative polarization of one print image was horizontal, and one was polarized laterally. In order to go from maximum intensity of one image to the maximum image of the other, the polarizer had to be rotated 90 degrees.

In method three, the greatest stress points appeared mainly around the edges of the plastic, near the joints. At these points, there was the most color, and the center of it was all black.

Method four had a polarizing angle of 56 and the polarization was determined to be horizontal. We determined this by looking at the picture on the front of the lab and finding out what the polarization was for the polarizer, and then looking through it at different angles (0 ,90 , 180 , 270 ). When looking at the ceiling light reflection, it was not polarized because there are so many rays coming from all different directions and interfering with the light directly above. Had we isolated it better, it may have then been polarized, but under the circumstances, there was too much light involved.

Using the polarizer, in method five, we were able to confirm the prediction about scattered light given in the lab manual. The light was dark when the polarizer was at 0 and 180 . When the polarizer was at 90 and 270 , the light coming through was bright. The scattered light is horizontally polarized.

In part two, method one, my curve again had larger values than the calculated values. I feel this could have occurred because not only could we have been more precise, however, our light wasn’t the only light in the room and there could have been some interference from the other light sources. For a real light bulb, I would expect the inverse square law to fail atdistances because
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