POLARIZATION AND BIREFRINGENCE

Transcription

1 UNIT 3 POLARIZATION AND BIREFRINGENCE Name Lab Partner(s) Date Lab Section # TA signature Be sure to have your TA check your lab work and sign this sheet before you leave. Save it until the end of the semester. You may need it for studying, and it is your record of having done the lab. (A) Malus Law See lab manual for instructions. θ is the angle with respect to the first polarizer. I 0 is the intensity emerging from the first polarizer. In what way are your observations consistent with Malus Law? (B) Third Polarizer between two Crossed Polarizers First polarizer at 0 0, second at 90 0, third polarizer between them at angle θ to the first. (B.1) Sketch of observed transmitted intensity as a function of θ 1

2 (B.2) With the help of a diagram of polarization vectors, explain why light can pass this combination of three polarizers. (B.3) Write an expression of the transmitted intensity as a function of θ. I 0 is the intensity emerging from the first polarizer. What is the maximum possible transmitted intensity? (C) Brewster s Angle Sketch your arrangement. Explain which measurement you took to find θ Β. θ Β = n glass = Suppose you angle measurement were off by 5 0. By how much would that change your result for the index of refraction of the glass? 2

3 (D) Birefringence and Polarization Practical Note: In many of the following experiments, starting with (1b), you will handle a stack consisting of a polarizer, a birefringent plate, a protractor, and another polarizer (analyzer). The analyzer on top of this stack needs to be rotated. Hold the first polarizer, the birefringent plate, and the protractor in one hand, with the protractor on top. When you rotate the analyzer on top of this stack with your other hand, you won t disturb the orientation of the lower pieces which are firmly held and protected by the protractor. (D.1) (a) Find the orientation of the slow and fast axes of the birefringent plates (you can t determine which is which, but you know they are perpendicular to each other in this material). Hint: If the incident light is polarized in the direction of either the s or the f axis, what effect will the birefringent material have on the polarization? Describe your procedure: The orientation of the s and f axis with respect to the edges of the birefringent plates is: (b) Identify the ¼ - wave plate Describe your procedure and reasoning: (c) Identify the ½ - wave plate Describe your procedure and reasoning: 3

4 (D.2) (a) ¼ -wave plate. Sketch the results of your polarization tests. θ is the angle of the second polarizer (analyzer) with respect to the first polarizer. If the transmitted light is linearly polarized, give the angle, α, with respect to the first polarizer. If it is elliptically polarized, at what angles are the major axes of the ellipse? Careful: your protractor may read 90 0 when you call it 0 0. s or f axis at 0 0 to polarization of incident light s or f axis at 30 0 to polarization of incident light s or f axis at 45 0 to polarization of incident light s or f axis at 90 0 to polarization of incident light. 4

5 (D.2) (b) 1/2 -wave plate. Sketch the results of your polarization tests. θ is the angle of the second polarizer (analyzer) with respect to the first polarizer s or f axis at 0 0 to polarization of incident light Polarization state of light after passing birefringent plate: α = s or f axis at 30 0 to polarization of incident light Polarization state of light after passing birefringent plate: α = Explain with a vector diagram.. s or f axis at 45 0 to polarization of incident light Polarization state of light after passing birefringent plate: α = Explain with a vector diagram. s or f axis at 90 0 to polarization of incident light. Polarization state of light after passing birefringent plate: angle: 5

6 (E) Laminate of ¼-wave Plate and Polaroid (E.1) Refer to lab manual. Polarization state: Experimental check:: (E.2) Polarization state: Experimental check: (E.3) Explanation of laminate plus mirror trick : (F) Stress-Induced Birefringence Note that the index of refraction can depend on wavelength; this is responsible for color effects, both in the stressed piece of Lucite and the stretched tape. Brief description and explanation of observations: 6