lab polarize

# lab polarize - Unit 3 Polarization and Birefringence...

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Unit 3 Polarization and Birefringence Introduction Phenomena in which polarized light plays a part are frequently surprising, often beautiful, and of considerable practical use. The series of experiments suggested below permits observation of three schemes for producing polarized light -- by use of selective absorption (Polaroids), by reflection, and by use of doubly refracting (birefringent) materials. (For birefringent materials, the index of refraction has different values for the two senses of polarization of light.) Most of your measurements will be qualitative or semi-quantitative, but do make careful notes of what you observe. Given that these topics are often not covered in lecture, we have provided an appendix to discuss the relevant physics concepts of polarization and birefringence. Most texts do a credible job on polarization, so our discussion of that will be brief in that appendix. Equipment Description You will need two white light sources, a simple desk lamp or bulb for most of the studies, and a penlight when you need a more directed “beam”. You should have a protractor for measuring angles and a ruler for measuring lengths. In addition there is a “kit” that contains, among other things: • several pieces of Polaroid film, at least two of which are rectangular, having their pass axis along one of the edges • two simple pieces of birefringent material that will look identical, except one is thicker than the other • a piece of Polaroid that has a birefringent sheet laminated to it • a small mirror • a small sheet of flat glass • a Lucite shape for investigating stress lines • some stretched cellophane tape Experiments (A) Look at a light source through two superposed sheets of Polaroid, and note how the intensity varies as they are rotated relative to each other. Sketch how the transmitted intensity varies as one Polaroid is rotated relative to the other through 360 ° . Does your observation agree with the theory of "Malus' Law", which is that I = I 0 cos 2 θ ? (B) Between two crossed Polaroids ( i.e ., θ = 90 ° so that the throughput is minimal) insert a third Polaroid sheet and rotate this third one without disturbing the other two. Sketch the transmitted intensity as the middle Polaroid is turned through 360 ° .

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Give an explanation of how the insertion of the middle Polaroid can permit light to pass through the system. (C) Originally unpolarized light which has been reflected from an interface is found to be partially or completely polarized. For dielectric materials, there is an angle of incidence for which the reflected light is expected to be completely polarized. This angle is called Brewster's angle , θ B , and it is found that tan θ B = n 2 /n 1 where n 1 and n 2 are the indices of refraction of the medium from which the light comes and the medium into which it is directed. [Your textbook has a nice description of this phenomenon!] Devise a simple experimental setup, using the pen-light as a source and one of the
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lab polarize - Unit 3 Polarization and Birefringence...

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