NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY UNIT II-a POLARISATION

Transcription

1 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY UNIT II-a 1 POLARISATION SYLLABUS :Polarization: Introduction, Malus s law, double refraction, Nicol prism, Quarter wave and half wave plates. 1. Distinguish between polarized and un polarized light. (May 2003, June 2004) 2. How the polarized light is different from ordinary light? (J 2017) S.No. UN POLARIZED LIGHT POLARIZED LIGHT 1 An un polarised beam consists of large number of rays having vibrations of electric vector in all planes with equal probability at right angles to the direction of propagation. It can also be considered as light beam having two electric vectors vibrating at right angles. This can be split into two plane polarised waves. In a polarised beam the electric vector vibrates in one plane only. 2 When an unpolarised beam is passed through a polaroid the intensity of the transmitted light does not change on rotating the Polaroid. When a polarised beam is passed through a polaroid the intensity of the transmitted light changes from maximum to zero on rotating the Polaroid. 3. Define a) Plane of vibration b) Plane of polarisation (2003,04,05,06) Plane of vibration: the plane in which the vibration of electric vector is confined. This plane contains the direction of vibration and direction of propagation. Plane of polarisation: the plane at right angles to the plane of vibration and containing the directions of propagation is called plane of polarisation. 4. What is Brewster s law? Explain how this can be used to find the polarizing angles of various crystals. (June 2004, 2006) Whenever an unpolarised beam is reflected from a denser medium it will be partially polarised. On changing the angle of incidence it is found that for a certain angle of incidence the reflected light is plane polarised and the refracted light is partially polarised. This angle of incidence is called polarising angle (i p ) or Brewster s angle (θ B ). The polarising angle depends on the refractive index of the denser medium. Brewster found that tan (i p ) = μ

2 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY Brewster s law: the tangent of the angle of polarisation is numerically equal to the refractive index of the medium. tan (i p ) = μ 2 5. Starting from Brewster s law show that the refracted and reflected rays are at right angles. 6. Prove that if the angle of incidence corresponds to the Brewster s angle, then the angle between reflected and refracted beam is 90.(2005) From Brewster s law tan (i p ) = μ...(1) where i p is the angle of incidence called polarising angle. sin ip sin r From Snell s law μ......(2) where r is the angle of refraction sin i p p From equations (1) and (2) we get......(3) From equation (3) we get sin r = cos i p = sin (90 - i p )...(4) From equation (4) we get r = 90 o i p Or i p + r = 90 o Hence the refracted and reflected rays are at right angles. cosi p sin i sin r 7. Derive Brewster s law Brewster s law: the refractive index of the material medium (μ) is equal to tangent of the angle of polarisation i p. μ = tan i p. Whenever a ray of light is incident at polarising angle it is found that the reflected and refracted rays are perpendicular. i.e., i p + r = 90 o sin ip sin r From Snell s law μ......(1) But r = 90 o i p sin i Hence p μ......(2) o sin (90 i ) p But sin (90 o i p ) = cos i p sin i p Therefore equation (2) becomes μ......(3) cos i p Or tan (i p ) = μ...(4) Problem 1: find the polarising angles and corresponding angles of refraction if the refractive index of the medium is 1) 1.5 2) ) 3 4) 2 8. What is the relation between polarising angle and critical angle? We know that the refractive index and polarising angle are related as μ = tan (i p )...(1) We also know that the refractive index and critical angle are related as μ = 1/sin(C)...(2) From equations (1) and (2) we get tan (i p ) =1/sin(C)

3 3 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY Problem 1: The polarising angle of a medium is 60 o. Find its refractive index and critical angle. Problem 2: The critical angle of a medium is 45 o. Find its refractive index and polarising angle. Problem 3: The polarising angle of a medium is 60 o. Find its refractive index and angle of refraction. 9. Explain a) circularly polarised light b) elliptically polarised light c) partially polarised light a) Circularly polarised light: in this light the electric vector of constant magnitude rotates while proceeding in the forward direction creating an impression of a circle. Sideways it appears like a helix. b) Elliptically polarised light: in this light the electric vector of variable magnitude rotates while proceeding in the forward direction. The magnitude of the electric vector varies periodically giving an impression of an ellipse. Sideways it appears like a helix. c) Partially polarised light: it is a mixture of plane polarised light and a small extent of unpolarised light. 10. What is pile of plates. When unpolarised light is incident at polarising angle the reflected light is completely plane polarised light and the transmitted light is partially polarised. If the process is repeated by using a number of plates at polarising angle the transmitted light will also become plane polarised. This arrangement is called pile of plates. 11. What is meant by double refraction? Explain with one example When un polarised light beam is passed through certain crystals like calcite or quartz it is found that it splits into two rays. One of them obeys Snell s law. It is called ordinary ray or o-ray. The other ray does not obey Snell s law. It is called extraordinary ray or e-ray. It is further found that they are plane polarised in perpendicular planes. Example: Suppose a mark is made on a white paper and a calcite crystal is placed over it. On looking from the top we observe two images. On rotating the crystal one of the images remains stationary and the other moves in a circle. The stationary image is ordinary image and the other is called extraordinary image. 12. Explain briefly optic axis. It is the direction along which both o-ray and e-ray travel with the same velocity. The crystal has same refractive index for both the rays. The difference in their velocities and refractive indices will be maximum at right angles to the optic axis. In uniaxial crystal (calcite, quartz,

4 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY tourmaline ) we have only one optic axis whereas in biaxial crystals (topaz) there will be two optic axes What are a) Positive crystals b)negative crystals. S.No. Positive crystals Negative crystals 1. The e-ray travels slower than o-ray in all directions except along the optic axis i.e., Ve < Vo axis i.e., Vo < Ve 2 The refractive index for e-ray is larger than that for o-ray in all directions except along the optic axis. i.e., μe > μo 3 The wave front for o- ray encloses that for e-ray 4 Example: quartz Example: calcite The o-ray travels slower than e-ray in all directions except along the optic The refractive index for o-ray is larger than that for e-ray in all directions except along the optic axis i.e., μe < μo The wave front for e- ray encloses that for o-ray 14. Discuss the construction of a Nicol prism (June 2005) Principle: Nicol prism is a device for producing and analysing a plane polarised light. When an ordinary light is transmitted through a calcite crystal, it splits into ordinary and extra-ordinary beams which are completely plane polarised with vibrations in two mutually perpendicular planes. Nicol eliminated the ordinary beam by utilising the phenomenon of total reflection at a thin film of Canada balsam separating the two pieces of calcite. Construction: A calcite crystal whose length is three times as its width is taken. The end faces of this crystal are grounded in such a way that the angles in the principal section become 68 and 112 instead of 71 and 109. The crystal is cut in two pieces. The two cut surfaces are polished and then cemented together by Canada balsam. The refractive index of Canada balsam lies between the refractive indices for the ordinary and extra-ordinary rays for calcite. For sodium D lines, the values are given below: Refractive index for ordinary μo =

5 5 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY Refractive index for Canada balsam μ = 1.55 Refractive index for extra-ordinary μe = Action: When a beam of light enters the face PM in direction parallel to the long side, it is doubly refracted into ordinary plane polarised beam and extra-ordinary plane polarised beam. The Canada balsam acts as a rarer medium for the ordinary ray and denser medium for the extraordinary ray. The ordinary ray is completely reflected at Canada balsam surface and is absorbed by the black paint on the Nicol prism. The extraordinary ray is thus transmitted with no appreciable loss in intensity. It is slightly displaced laterally but emerges out of the prism parallel to its original direction. Thus only the extra-ordinary ray which is plane polarised is transmitted. 15. Explain 1) quarter wave plate and 2) half wave plate 1) QUARTER-WAVE PLATE It consists of calcite crystal cut with its face containing optic axis. The thickness of the calcite crystal plate is such that it introduces a phase difference of π/2 or a path difference of λ/4 between ordinary and extra-ordinary waves. For a quarter wave plate (μ o - μ e )t = λ/4 or t = λ/4(μ o - μ e ) Action: A quarter wave plate is used to convert plane polarised light into circularly polarised or elliptically polarised light and vice versa. 2) HALF-WAVE PLATE it consists of calcite crystal cut with its face containing optic axis. The thickness of the calcite crystal plate is such that it introduces a phase difference of π or a path difference of λ/2 between ordinary and extraordinary waves. For a half wave plate (μ o - μ e )t = λ/2 or t = λ/2(μ o - μ e ) Action: A half wave plate is used to change the plane of vibration of a plane polarised light. Problem 1: Plane polarised light is incident on a piece of quartz-cut parallel to the axis. Find the least thickness for which the ordinary and extra-ordinary rays combine to form, plane polarised light. Given μ o = , μ e = and λ = 5 x 10-5 cm Hint: For half wave plate t = λ/2(μ e μ o ) Problem 2: Calculate the thickness of a mica sheet required for making a quarter wave plate for λ = 5460 Å. The indices of refraction for the ordinary and extra-ordinary rays in mica are and Hint: For a quarter wave plate t = λ/4(μ e μ o ) Problem 3: Calculate the thickness of (i) a quarter wave plate and (ii) half wave plate. Given that μ o = 1.533, μ e = and λ = 5400Å

6 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY 16. Explain polarisation by a selective absorption or dichroism. Dichroism: certain doubly refracting crystals have the property of absorbing either o- ray or e-ray to a greater extent than the other. The cystals showing this property are termed as dichroic and the phenomenon is known as dichroism or selective absorption. It is shown by tourmaline. Tourmaline absorbs ordinary ray much more strongly than the extra-ordinary ray. Consider a plate of tourmaline cut with its face parallel to the optic axis. Suppose unpolarised light is incident on it, the light splits into ordinary and extra-ordinary rays. Both the rays are plane polarised and travel through the crystal in the same direction. When the plate is sufficiently thick, the ordinary ray is almost completely absorbed and the extra-ordinary ray is transmitted. In this way a plane polarised light is obtained What are polaroids. What are their uses. Polaroids: The phenomenon of selective absorption is utilized in the construction of polaroids. Herapath was successful in producing synthetic dichroic crystal of iodo sulphate of quinine called Herapathite after his name. A more recent type of polaroid is H-polaroid developed by E.H.Land. This polaroid is prepared by stretching a film of polyvinyl alcohol three to eight times of its original length. The stretching orients the molecules with their longer axes in the direction of stress and the material under stress becomes doubly refracting. It acts as dichroic crystal when impregnated with iodine.the iodine atoms provide electrons which can oscillate along the chain. These will absorb the electric vector parallel to the chains. If the stretched film of polyvinyl alcohol is heated in presence of a strong dehydrating agent, it becomes strongly dichroic and is very stable. This polaroid is called as K-polaroid. Uses of polaroids. (1) They can be used to produce and analyse plane polarised light. (2) The polaroids are of a relatively low cost than Nicols. (3) They are used in sun-glasses to cut-off the glare produced by light reflected from polished and semi smooth surfaces. (4) Polaroids are used to control the intensity of light in trains and aeroplanes. (5) Polaroid glasses are used to produce as well as to watch 3-D movies. 6) They are used to eliminate the head light glare in motor cars. 18. Explain polarisation by scattering When light is scattered by gas molecules or particles very small in size it is found that the scattered light is partially polarised. The intensity of scattered light is found to be inversely proportional to the fourth power of wave length I α 1/λ 4.

7 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY 19. Explain why the sky appears blue and red at different times during a day time When Sun rays pass through the atmosphere they undergo scattering. The shorter wavelengths like violet, blue undergo scattering at larger angles. Hence when we look above the sky appears blue. The longer wavelengths like red, orange undergo least scattering. Hence at the sunrise and sunset the sky appears brown State Malus law When a plane polarised light of intensity (I 1 ) is incident on a Polariser the intensity of the transmitted light (I θ )is proportional to the square of the cosine of the angle (θ) between the plane of polarisation of light and the transmission axis of the polariser I θ = I 1 cos 2 θ Problem: a polarised beam of intensity Io is incident on a Polaroid at an angle 1)0 o 2) 30 o 3) 45 o 4) 60 o 5) 90 o 21. Explain Malus law Suppose a plane polarised light of intensity Io and electric vector of amplitude E o is incident at an angle θ with the transmission axis. The component E y = E o cos θ is transmitted. The intensity of the transmitted light I y = E 2 y = E 2 o cos 2 θ or I y = I o cos 2 θ POLARISATION OF LIGHT OBJECTIVE 1. In an unpolarised beam the electric vector vibrates in all directions perpendicular to the direction of propagation 2. If the vibrations of electric vector are confined along a single direction, perpendicular to the direction of propagation of light, then the light is said to be plane polarised or linearly polarized, and the phenomenon is called polarisation. 3. The plane containing the direction of vibration or the plane in which the electric vector vibrates is known as plane of vibration. 4. The plane in which no vibration occurs or the plane in which the component of electric vector is zero is called plane of polarisation. 5. Plane of polarisation and plane of vibration are always perpendicular to each other. 6. Light waves can be polarized 7. Polarisation is convincing proof of transverse nature of light wave. 8. An unpolarised light is equivalent to the superposition of two mutually perpendicular identical plane polarised lights. 9. Light waves are electromagnetic waves with electric and magnetic vectors. Our eyes are sensitive to the electric vector.

8 8 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY METHODS OF OBTAINING PLANE POLARISED LIGHT 10. By reflection 2) By refraction 3) By double refraction 4) By dichroism 5) By scattering By reflection : 11. Brewster observed that when ordinary light is incident on a transparent substance, at a particular angle, the reflected light is completely plane polarised with vibrations in a plane perpendicular to the plane of incidence. This specific angle of incidence (ip) is known as polarising angle. 12. Brewster law: The tangent of polarising angle is numerically equal to the refractive index. Tan(i p ) = μ or i p = tan -1 (μ) 13. Polarizing angle for glass is 57.5 o. 14. A transparent medium also has critical angle C given by sin C = 1/μ 15. Polarising angle and critical angle are related as or tan(i p ) = 1/sin C By refraction: 16. By repeated refractions the ray can be made fully polarized (pile of plates) By Double Refraction: 17. When an object is seen through calcite /quartz crystal two images of that object can be seen. If the crystal is rotated the image due to E-ray rotates around the image due to O-ray. 18. O- ray obeys Snell s law and e- ray does not obey Snell s law. 19. Both ordinary and extraordinary rays are plane polarised in perpendicular planes. 20. In a negative crystal is Ve greater than Vo and µe is less than µo at right angles to optic axis. 21. In a positive crystal is Ve less than Vo and µe is more than µo at right angles to optic axis. 22. In a double refracting crystal along optic axis Ve is equal Vo and µe is equal µo 23. Calcite is a negative crystal. 24. Quartz is positive crystal. 25. Nicol prism produces polarised light by double refraction. 26. In Nicol prism only extraordinary ray is transmitted while ordinary ray undergoes total internal reflection. 27. In Nicol prism the Canada balsam is rarer for ordinary ray, so it gets reflected. 28. In Nicol prism the Canada balsam is denser for ordinary ray, so it gets transmitted. 29. The refractive index of Canada balsam for λ = 5893 Ǻ is In Nicol prism the refractive index for e ray varies between and Nicols prism can be used as polariser and analyser. 32. A quarter wave plate introduces a path difference of λ/4 between o-ray and e-ray. λ 33. The thickness of a quarter wave plate is t = 4 μ o μ e 34. A quarter wave plate is used to convert plane polarized light into circularly/elliptically polarized light or vice versa. 35. A half wave plate introduces a path difference of λ/2 between o-ray and e-ray.

9 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY λ 36. The thickness of a half wave plate is t = 2 μ o μ e 37. A half wave plate is used to rotate the plane of polarization of plane polarized light. 38. By Dichroism (selective absorption) 39. Dichroism is the property of unequal absorption of ordinary and extraordinary rays by some crystals. 40. tourmaline is dichroic. 41. iodosulphate of quinine is dichroic 42. If unpolarised light passes through proper thickness of the crystal showing dichroism, the transmitted light will be plane polarised. 43. Polaroids work on this principle of dichroism. 44. If an unpolarised light is converted into plane polarised light by passing it through a polaroid or polariser, intensity of transmitted light is half of the intensity of incident light. I = ½ Io. The amplitude of the transmitted light A = 1 Ao An unknown light is passed through a Polaroid which is rotated. If the intensity changes from maximum to zero, then the light is plane polarized. 46. An unknown light is passed through a Polaroid which is rotated. If the intensity changes from maximum to minimum, then the light is partially polarized or elliptically polarised If the intensity does not change, the light is either unpolarised or circularly polarised By scattering : 48. The scattered light in directions perpendicular to the direction of incident light is completely plane polarised while transmitted light is partially polarised. MALUS LAW 49. The intensity of the polarised light transmitted through the analyser varies as the square of the cosine of the angle between the plane of transmission of the analyser and the plane of polariser. 50. If θ is the angle between the two transmission planes at any instant, the transmitted component from analyser is given by I θ = I cos 2 θ intensity of the light emerging from the polariser. 51. USES OF POLAROIDS 1) in the production and watching 3D films 2) in the production and analysis of polarized light. 3) in controlling intensity of light. 5) to study optical activity of certain organic substances. 6) to study the helical (spring like) structure of neucleic acids. 52. When unpolarised beam is reflected from a transparent surface at polarising angle, the reflected beam is..and the refracted beam is 53. When a beam is incident at 53 the reflected beam is completely polarised. The angle of refraction is. 54. For a certain medium the polarising angle is 60. Its refractive index is and critical angle is.

10 10 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY 55. For a certain medium the critical angle is 45. Its refractive index is and polarising angle is An unpolarised light is incident on a plate of refractive index 3 and the reflected light is found to be completely plane polarised. The angles of incidence and refraction are..respectively 57. The refractive index of a certain flint glass is 5/3. For what incident angle is the light reflected from the surface of the glass completely polarized if the glass is immersed in water The amplitude of the unpolarised light incident on a polartser is A. The amplitude of the polarised light transmitted through it is 59. When an unpolarised light of intensity Io is incident on a polarising sheet, the intensity of light which is transmitted is. 60. The axes of the polariser and analyser are inclined to each other at 45. If the amplitude of the unpolarised light incident on the polariser is A, the amplitude of the light transmitted through the analyser is 61. Unpolarized light of intensity Io is incident on a polarizer and the emerging light strikes a second polarizing filter with its axis at 45 to that of the first. Determine the intensity of the emerging beam and its state of polarization 62. A beam of ordinary light is incident on a system of three polaroids which are arranged in succession such that each polaroid is turned through 30 0 with respect to the, preceding one. The percentage of the incident intensity that emerges out from the system is approximately 63. A beam of ordinary light is incident on a system of three polaroids which are arranged in succession such that each polaroid is turned through 45 0 with respect to the, preceding one. The percentage of the incident intensity that emerges out from the system is approximately A beam of ordinary light is incident on a system of four polaroids which are arranged in succession such that each polaroid is turned through 30 0 with respect to the, preceding one. The percentage of the incident intensity that emerges out from the system is approximately. 65. A beam of unpolarised light is incident on a tourmaline crystal. The intensity of the emergent light is I 0, The emergent light is incident on another tourmaline crystal. It is found that no light emerges out of the second crystal. If now, the first crystal is rotated through 45, the intensity of the light emerging through the second crystal will be Transverse wave nature is established by (1) Interference (2) Diffraction (3) Polarization (4) All the above 67. Which phenomenon causes the polarisation of light (a) reflection (b) double reflection (c) double refraction (d) diffraction Which of the following phenomenon is not common to sound and light waves (1) Interference (2) Diffraction (3) Polarisation (4) Reflection 68. Helical structure of nucleic acids can be studied using the phenomenon of (1) Interference (2) Diffraction (3) Polarisation (4) Dispersion

11 11 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY 69. Polarisation can be produced by (1) Reflection (2) Double refraction (3) Scattering (4) All of the above 70. Plane polarised light can be produced by (a) simple reflection (b) Nicol s prism (c) pile of plates (d) all of the above 71. The tangent of polarizing angle is numerically equal to (1) diversity of the reflecting medium (2) refractive index of the reflecting medium (3) velocity of light in reflecting medium (4) elastic modulus of reflecting medium 72. A calcite crystal placed over an ink dot is rotated. On seeing through the crystal one finds (1) Two stationary dots (2) Two dots moving along parallel straight lines (3) One dot rotating about the other (4) both dots rotating about a common axis 73. In double refraction (1) only the 0-ray is polarised (2) only the E-ray is polarized (3) both 0-ray and E-ray are polarised (4) neither 0-ray nor E-ray is polarised 74. Nicol prism is based on the action of (a) refraction (b) double refraction (c) Scattering (d) none of the above 75. Dichroism is property where (1) unequal absorption of ordinary and extraordinary rays takes place (2) equal absorption of ordinary and extra ordinary rays taken places (3) plane of polarization rotates (4) None of the above 76. Which of the following exhibits dichroism (1) Quartz 2) Crown glass (3) Tourmaline (4) All the above 77. Which of the following is dichroic (1) poly vinyl alcohol (2) quartz (3) calcite (4) diamond 78. Polaroids are used (1) to eliminate head light glare in automobile (2) in production of 3-D motion pictures (3) in sun glasses 4) all the above 79. The synthetic material used in polaroids possess the property of (1) Anomalous thermal expansion (2) Optical activity (3) Dichroism (4) None of the above 80. Polaroid sunglasses are preferred because they (1) reduce the intensity of light (2) have soothing colours (3) are cheaper (4) can change colours 81. The angle of incidence at which reflected light is totally polarized for a day travelling from air to glass (refractive index n), is (1) Sin 1 (n) (2) Sin 1 ( 1 n ) (3) Tan 1 ( 1 n ) (4) Tan 1 (n)

12 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY 82. The critical angle of a transparent crystal is Then its polarizing angle is (1) = tan 1 ( 2) (2) = sin 1 ( 2) (3) = cos 1 ( 1 2 ) (4) = cot 1 ( 2) 83. If unpolarised light is incident on a crystal at 60 0 so that the reflected light is completely polarised, then refractive index of the crystal should be (1) 2 (2) 1 (3) 3 (4) When an unpolarised light of intensity I 0 is incident on a polarizing sheet, the intensity of light which does not get transmitted is (1) 0 (2) I 0 (3) 1 Io (4) I The critical angle of a certain medium is sin 1 ( 3 ). The polarizing angle of the 5 medium is (1) sin 1 ( 4 5 ) (2) tan 1 ( 5 3 ) (3) tan 1 ( 3 4 ) (4) tan 1 ( 4 3 ) 86. Critical angle for certain medium is sin 1 (0.6). The polarizing angle of that medium is (1) tan 1 (1.5) (2) sin 1 (0.8) (3) tan 1 (1.6667) 4) tan 1 (0.6667) 87. When the angle of incidence on a material is 60 o, the reflected light is completely polarized. The velocity of the refracted ray inside the material is (in ms 1 ) 12 (1) ) [ 3 2 ] 108 (3) (4) In case of linearly polarized light, the magnitude of the electric field vector (1) does not change with time (2) varies periodically with time (3) increases and decreases linearly with time (4) is parallel to the direction of propagation 89. A ray of light is incident at polarising angle such that its deviation is 24 o, then angle of incidence is (1) 24 o (2) 57 o (3) 66 o (4) 90 o 90. An unpolarised beam of intensity I 0 is incident on a pair of Nicols making an angle of 60 o with each other. The intensity of light emerging from the pair is (1) I 0 (2) I 0 (3) I 0 (4) I Which of the following is a dichroic crystal? (1) Quartz (2) Tourmaline (3) Mica (4) Selenite 92. An unpolarised beam of intensity I 0 falls on a polaroid. The intensity of the emergent light is (1) I 0 (2) I 0 (3) I 0 (4) zero When an unpolarized light of intensity I 0 is incident on a polarizing sheet, the intensity of the light which does not get transmitted is (1) 1 2 I0 (2) 1 4 I 0 (3) zero (4) I 0

13 13 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY 94. The angle of incidence of light is equal to Brewster s angle, then (A) reflected ray is perpendicular to refracted ray (B) refracted ray is parallel to reflected ray (C) reflected light is polarized having its electric vector in plane of incidence (D) refracted light is polarized (1) (A) and (D) are true (2) (A) and (B) are true 3) (A) and (C) are true (4) (B) and (C) are true 95. Brewster s angle in terms of refractive index ( ) of the medium (1) tan 1 μ (2) sin 1 (3) sin 1 μ (4) tan Sound wave in air cannot be polarized because (1) their speed is small (2) they require medium (3) these are longitudinal (4) their speed is temperature dependent 97. In case of linearly polarised light, the magnitude of the electric field vector (1) does not change with time 2) varies periodically with time (3) increases and decreases linearly with time (4) is parallel to the direction of propagation 98. When unpolarised light beam is incident from air onto glass (n=1.5) at the polarising angle (1) reflected beam is polarised 100 percent (2) reflected and refracted beams are partially polarised (3) the reason for (a) is that almost all the light is reflected (4) All of the above 99. For the study of the helical structure of nucleic acids, the property of electromagnetic radiation, generally used is (1) reflection (2) interference (3) diffraction (4) polarization 100. In polarized light the angle between the plane of polarization and plane of vibration is (1) 30 o (2) 45 o (3) 60 o (4) 90 o 101. A ray of light is incident on the surface of a glass plate at an angle of incidence equal to Brewster s angle. If represents index of glass with respect to air then the angle between the reflected and the refracted rays is (1) (90+ ) (2) 90 o (3) sin 1 (4 cos ) (4) sin 1 ( sin ϕ ) Polaroid sun glasses decrease glare on a sunny day because they (a) block a portion of light (b) have a special colour (c) completely absorb the light, (d) refract the light 103. In elliptically polarised light (a) amplitude of vibrations chages in direction only, (b) amplitude of vibrations changes in magnitude only (c) amplitude of vibrations changes in magnitude and direction both (d) none of the above When two Polaroid sheets are crossed perpendicular to each other (1) light gets completely transmitted (2) light gets completely extinguished (3) light causes harm to the human eye (4) alternate bright and dark bands can be seen 105. Unpolarising light falls on two polarizing sheets oriented that no light is

14 14 NAWAB SHAH ALAM KHAN COLLEGE OF ENGINEERING & TECHNOLOGY transmitted. If a third polarizing sheet is placed between them; not parallel to either of two sheets in question. (1) no light is transmitted (2) some light is transmitted (3) light may or may not be transmitted (4) certainly 50% light is transmitted 106. Plane polarized light is passed through a Polaroid. Now the Polaroid is given one complete rotation about the direction of light propagation. When viewed through the Polaroid, one of the following is observed. (1) The intensity of light gradually decreases to zero and then remains as zero. (2) The intensity of light becomes maximum twice and zero twice (3) The intensity of light becomes maximum and stays maximum. (4) Intensity of light does not change Choose the correct statement: (1) Brewster s angle is independent of wavelength of light. (2) Brewster s angle is independent of nature of reflecting surface. (3) Brewster s angle is different for different wavelengths of light. (4) Brewster s angle is depends of wavelength of light but is independent of reflecting surface If the critical angle of a medium is 45 o, its angle of polarisation is (1) tan 1 ( 1 2 ) (2) tan 1 ( 2) (3) 45 o (4) 57 o 109. Two polarising sheets have their polarizing directions parallel to each other so that the intensity of transmitted light is maximum. Through what angle must either sheet be turned so that the intensity becomes one half the initial value? (1) 60 o, 120 o 2) 45 o, 135 o (3) 30 o, 135 o (4) 0 o, 180 o 110. A beam of unpolarised light is incident on a tourmaline crystal C 1. The intensity of the emergent light is I 0 and it is incident on another tourmaline crystal C 2. It is found that no light emerges from C 2. If now C 1 is rotated through 45 o towards C 2, the intensity of the light emerging from C 2 is (1) Zero (2) I 0 (3) I 0 (4) 3I When an unpolarised light propagating in air strikes the surface of a transparent medium at a glancing angle, the reflected and refracted rays are found to be mutually perpendicular. The refractive index of the medium is equal to (1) tan ( ) (2) tan (90 o ) (3) cot ( ) (4) cot (90 o )