4.4 Polarisation [26 marks]

Transcription

1 4.4 Polarisation [26 marks] 1. Unpolarized light of intensity I 0 is incident on the first of two polarizing sheets. Initially the planes of polarization of the sheets are perpendicular. Which sheet must be rotated and by what angle so that light of intensity I0 can emerge from the second sheet? 4 C 2. A beam of unpolarized light is incident on the first of two parallel polarizers. The transmission axes of the two polarizers are initially parallel. The first polarizer is now rotated about the direction of the incident beam by an angle smaller than 90. Which gives the changes, if any, in the intensity and polarization of the transmitted light? A

2 3. Horizontally polarized light of intensity I 0 enters a polarizer P whose polarization axis makes an angle of θ degrees with the horizontal. Light from P is then incident on a polarizer A with fixed vertical polarization axis. The angle θ is varied from 0 to 90 degrees. Which of the following represents the variation with θ of the intensity I of the light transmitted through A? B

3 4. An unpolarized ray of light in air is incident on the surface of water. The reflected ray is completely polarized. Which of the following are separated by an angle of 90? A. The incident ray and the reflected ray B. The reflected ray and the refracted ray C. The refracted ray and the incident ray D. The refracted ray and the surface of the water B 5. Two polarizers have polarizing axes that make an angle of 30 to each other. Unpolarized light of intensity I 1 is incident on the first polarizer so that light of intensity I 2 emerges from the second polarizer, as shown below. 3 I 1 The cosine of 30 is. What is the ratio? 2 A. B. C. D I2 D

4 This question is about polarized light. An analyser is used with polarized light. 6a. Outline the function of an analyser in this context. [2 marks] by crossing the analyser with the polarized light; the angle of polarization/electric field vector can be determined; 6b. Polarized light of intensity I 0 is incident on the analyser. [3 marks] (i) The transmission axis of the analyser is at an angle of 25 to the electric field of the polarized light. Calculate, in terms of I 0, the intensity of the light that leaves the analyser. (ii) The angle θ between the transmission axis of the analyser and the electric field of the polarized light is varied. On the axes, sketch a graph to show the variation with θ of the intensity of the light leaving the analyser.

5 (i) (I = I 0 cos 2 25º =) 0.82I 0 ; (ii) cos 2 shape; (allow negative intensities for this mark) max at 0º and 180º, zero at 90º; (allow non-cos 2 line for this mark) This question is about polarization. 7. State what is meant by polarized light. light in which the electric vector oscillates on one plane/direction; In part (a) the definition was often not specific enough, the idea that the electric field vector is oscillating rather than just light was often omitted.

6 This question is about polarization. 8. Distinguish between polarized light and unpolarized light. [2 marks] light is said to be (plane) polarized if the electric field (vector) lies on one plane; when light is unpolarized, the electric field (vector) lies on many planes/does not lie on a specific plane; (a) was either very well answered or lacking in reference to the electric field.

7 9. Unpolarized light of intensity I 0 is incident on a polarizer that has a vertical transmission axis. The polarizer is rotated by an angle θ about the direction of the incident light. The intensity of the transmitted light is I. Which graph correctly I shows the variation with the angle θ of the ratio? I0 D It would seem that candidates had not read this question carefully and were trying to remember similar past questions. The incoming light is unpolarized, hence B and C must be incorrect. As the polarizer will reduce the intensity, A must also be incorrect.

8 10. A person wearing polarizing sunglasses stands at the edge of a pond in bright sunlight. The surface of the pond is flat and the line of sight of the person makes an angle θ with the surface. The refractive index of the pond water is n. What is the value of θ for which the intensity of the sunlight reflected by the surface to the person s eye is a minimum? A. tan 1(n) B. cos 1 ( 1 n) C. cos 1(n) D. tan 1 ( 1 n) D This was very poorly done with over half of the candidates opting for A. Simple recall of the Brewster angle, which involves tan of an angle, should cause candidates to eliminate B and C (which most of them did). To choose between A and D, though, it is required to look at the situation as depicted (rather than jumping to conclusions based upon familiar diagrams). θ is the angle to the surface not the angle of incidence. So A must be incorrect.

9 Unpolarized light of intensity I is incident on a polarizer with a vertical transmission axis. The transmitted light is incident on a sheet I0 of material X. After transmission through X the intensity of the light is. 2 It is suggested that X could be I. a polarizer with vertical transmission axis II. a polarizer with horizontal transmission axis III. non polarizing glass. Which of the above suggestions is/are correct? A. I and III only B. I only C. II only D. II and III only A This was another question that elicited a number of critical comments from teachers, although the statistics showed two-thirds of the candidates choosing the response A. The amount of light reflected from a glass surface is negligible (and unquantifiable within the parameters of the question), so clearly A was the best response. 12. Unpolarized light is incident on the surface of a transparent medium. The reflected light is completely plane polarized. The refracted light will be A. unpolarized. B. partially plane polarized. C. completely plane polarized at right angles to the reflected light. D. completely plane polarized parallel to the reflected light. B Candidates found this question to be the most challenging on the paper. At the Brewster angle the reflected light is plane polarized but that does not mean that the transmitted (refracted) light is plane polarized. There will still be some light in the same plane of vibration as the reflected light but there will now be less of it. The transmitted light is therefore partially plane polarized.

10 A beam of unpolarized light is incident on the surface of a liquid and is partially reflected and partially refracted as shown below. 13. The reflected light is completely horizontally polarized. Which of the following is the refractive index of the liquid? A. tan 40 B. tan 50 C. D. sin40 sin50 sin40 cos50 B The majority of the candidates got the answer to this question but an almost equal number thought that the refractive index was given by the ratio of sines in option C. This shows clearly that candidates thought they were dealing with Snell's law and not Brewster's law.

11 14. Unpolarized light is incident on a polarizer. The light transmitted by the first polarizer is then incident on a second polarizer. The polarizing axis of the second polarizer is at 60º to that of the first polarizer. The intensity emerging from the second polarizer is I f. Which of the following correctly gives the intensity incident on the first polarizer? If A. 8 If B. 4 C. 4If D. 8If D It is reasonable to expect higher level physics candidates to know the value of cos30, and if they do not, they can always sketch the relevant triangle. The evidence from the statistics, though, would suggest that the majority of candidates either omitted to factor in the effect of the light passing through the first polarizer, or that they thought the intensity was reduced by the factor cos60º (rather than cos260º) on passing through the second polarizer.

12 This question is about polarization. Unpolarized light is directed towards two polarizers. The dashed lines represent the transmission axes of the polarizers. The angle θ between the transmission axes of the polarizers is initially 0 º. On the axes below, sketch a graph to show how the intensity I of the light emerging from the second polarizer varies with θ. 15. [2 marks]

13 general cos 2 shape; zero intensity at 90 and maximum at both 0 and 180 ; This question is about polarization. 16a. State what is meant by polarized light. light in which the electric field oscillates in the same plane;

14 Light of intensity I is incident on a polarizer. The transmission axis of the polarizer is vertical. The polarizer is rotated by an angle 16b. 0 θ about the direction of the incident light. The intensity of the transmitted light is measured for various angles θ. [4 marks] On the axes below, sketch graphs to show the variation of the transmitted intensity I with θ when the incident light is (i) horizontally polarized. (ii) unpolarized. (i) zero at 0 and 180 degrees; peak at 90 degrees; (ii) horizontal straight-line; through half the incident intensity;

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