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Chapter Ray Optics and Optical Instrument Q1. Focal length of a convex lens of refractive index

1.5 is 2 cm. Focal length of the lens when immersed in a liquid of refractive index of 1.25 will be [1988] (a) 10 cm (b) 2.5 cm (c) 5 cm (d) 7.5 cm Ans: (c) Q2. Ray optics is valid, when characteristic dimensions are [1989] (a) of the same order as the wavelength of light (b) much smaller than the wavelength of light (c) of the order of one millimeter (d) much larger than the wavelength of light Ans: (d) Q3. A ray is incident at an angle of incidence i on one surface of a prism of small angle A and emerges normally from the opposite surface. If the refractive index of the material of prism is P, the angle of incidence is nearly equal to 1989

(a)

(b)

(c)

(d) Ans: (c) Q4. A beam of monochromatic light is refracted from vacuum into a medium of refractive index 1.5, the wavelength of refracted light will be 1991 (a) dependent on intensity of refracted light (b) same (c) smaller (d) larger Ans: (c) Q5. Green light of wavelength 5460 Å is incident on an air-glass interface. If the refractive index of glass is 1.5, the wave length of light in glass would be (c = 3 × 108 ms–1) [1991] (a) 3640 Å

(b) 5460 Å (c) 4861 Å (d) none of the above Ans: (a) Q6. There is a prism with refractive index equal to root 2 and the refracting angle equal to 30°. One of the refracting surfaces of the prism is polished. Abeam of monochromatic light will retrace its path if its angle of incidence over the refracting surface of the prism is [1992] (a) 0° (b) 30° (c) 45° (d) 60° Ans: (c) Q7. Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is 3/2 is [1993] (a) 2 × 10–4 sec (b) 2 × 108 sec (c) 2 × 10–11 sec (d) 2 × 1011 sec Ans: (c) Q8. A point source of light is placed 4 m below the surface of water of refractive index 5/3 . The minimum diameter of a disc, which should be placed over the source, on the surface of water to cut off all light coming out of water is [1994] (a) infinity (b) 6 m (c) 4 m (d) 3 m Ans: (b) Q9. Angle of deviation (G) by a prism (refractive index = P and supposing the angle of prism A to be small) can be given by [1994]

(a)

(b)

(c)

(d) Ans: (a) Q10. Focal length of a convex lens will be maximum for [1994] (a) blue light (b) yellow light (c) green light (d) red light Ans: (d)

MockTime.com Q11. An achromatic combination of lenses is formed by joining [1995] (a) 2 convex lenses (b) 2 concave lenses (c) 1 convex and 1 concave lens (d) 1 convex and 1 plane mirror Ans: (c) Q12. The hypermetropia is a [1995] (a) short-sight defect (b) long-sight defect (c) bad vision due to old age (d) none of these Ans: (b) Q13. If two mirrors are kept inclined at 60° to each other and a body is placed at the middle, then total number of images formed is [1995] (a) six (b) five (c) four (d) three Ans: (b) Q14. A lens is placed between a source of light and a wall. It forms images of area A1 and A2 on he wall for its two different positions. The area of the source of light is [1995]

(a)

(b)

(c)

(d) Ans: (a) Q15. A convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together. What will be their resulting power? (a) + 6.5 D (b) – 6.5 D[1996] (c) + 7.5 D (d) –0.75 D Ans: (d) Q16. Light travels through a glass plate of thickness t and refractive index u. If c is the speed of light in vacuum, the time taken by light to travel this thickness of glass is [1996]

(a)

(b)

(c)

(d) Ans: (d) Q17. One face of a rectangular glass plate 6 cm thick is silvered. An object held 8 cm in front of the first face forms an image 12 cm behind the silvered face. The refractive index of the glass is [1996] (a) 0.4 (b) 0.8 (c) 1.2 (d) 1.6 Ans: (c) Q18. (a) (b) (c) (d) Ans: (c) Q19. An astronomical telescope has a length of 44 cm and tenfold magnification. The focal length of the objective lens is [1997] (a) 4 cm (b) 40 cm (c) 44 cm (d) 440 cm Ans: (b)

Q20. Aluminous object is placed at a distance of 30 cm from the convex lens of focal length 20 cm. On the other side of the lens, at what distance from the lens a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it? [1998] (a) 12 cm (b) 30 cm (c) 50 cm (d) 60 cm Ans: (c) Q21. Light enters at an angle of incidence in a transparent rod of refractive index n. For what value of the refractive index of the material of the rod the light once entered into it will not leave it through its lateral face whatsoever be the value of angle of incidence? [1998]

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MockTime.com (c) (d) Ans: (a) Q22. A plano-convex lens is made of material of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is [1999] (a) 50 cm (b) 100 cm (c) 200 cm (d) 400 cm Ans: (b) Q23. Wavelength of light of frequency 100 Hz [1999] (a) 2 × 166 m (b) 3 × 106 m (c) 4 × 106 m (d) 5 × 106 m Ans: (b) Q24. The refractive index of the material of the prism is root 3; then the angle of minimum deviation of the prism is [1999] (a) 30° (b) 45° (c) 60° (d) 75° Ans: (c) Q25. The radius of curvature of a thin plano-convex lens is 10 cm (of curved surface) and the refractive index is 1.5. If the plane surface is silvered, then it behaves like a concave mirror of focal length (a) 10 cm (b) 15 cm [2000] (c) 20 cm (d) 5 cm Ans: (a) Q26. A person is six feet tall. How tall must a vertical mirror be if he is able to see his entire length? (a) 3 ft (b) 4.5 ft (c) 7.5 ft (d) 6 ft Ans: (a) Q27. Rainbow is formed due to a combination of [2000] (a) dispersion and total internal reflection (b) refraction and absorption (c) dispersion and focussing (d) refraction and scattering Ans: (a) Q28. An air bubble in a glass slab (u= 1.5) is 5 cm deep when viewed from one face and 2 cm deep when viewed from the opposite face. The thickness of the slab is [2000]

(a) 7.5 cm (b) 10.5 cm (c) 7 cm (d) 10 cm Ans: (b) Q29.

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(b)

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(d) Ans: (a) Q30. A body is located on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance 'd' ahead of second wall, then the required focal length will be [2002] (a) only d/4 (b) only d/2 (c) more than d/4 but less than d/2 (d) less than d/4 Ans: (b) Q31. A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will [2003] (a) remain unchanged (b) become zero (c) become infinite (d) become small, but non-zero Ans: (c) Q32. The refractive index of the material of a prism is root2 and its refracting angle is 30°. One of the refracting surfaces of the prism is made a mirror inwards. Abeam of monochromatic light enters the prism from the mirrored surface if its angle of incidence of the prism is [2004] (a) 30°

MockTime.com (b) 45° (c) 60° (d) 0° Ans: (b) Q33. A beam of light composed of red and green rays is incident obliquely at a point on the face of rectangular glass slab. When coming out on the opposite parallel face, the red and green rays emerge from [2004] (a) one point propagating in the same direction (b) two points propagating in two different non-parallel directions (c) two points propagating in two different parallel directions (d) one point propagating in two different directions Ans: (c) Q34. A telescope has an objective lens of 10 cm diameter and is situated at a distance of one kilometer from two objects. The minimum distance between these two objects, which can be resolved by the telescope, when the mean wavelength of light is 5000 Å, is of the order of (a) 5 cm (b) 0.5 m [2004] (c) 5 m (d) 5 mm Ans: (d) Q35. The angular resolution of a 10 cm diameter telescope at a wavelength of 5000 Å is of the order of [2005] (a) 106 rad (b) 10–2 rad (c) 10–4 rad (d) 10–6 rad Ans: (d) Q36. A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The power in diopters of the combination is [2006] (a) 50 (b) infinite (c) zero (d) 25 Ans: (c) Q37. A microscope is focused on a mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again? [2006] (a) 4.5 cm downward (b) 1 cm downward (c) 2 cm upward (d) 1 cm upward Ans: (d)

Q38. The frequency of a light wave in a material is 2 × 1014 Hz and wavelength is 5000 Å. The refractive index of material will be [2007] (a) 1.50 (b) 3.00 (c) 1.33 (d) 1.40 Ans: (b)

Q39. [2007] (a) 2.4 × 108 m/s (b) 3.0 × 108 m/s (c) 1.2 × 108 m/s (d) 1.8 × 108 m/s Ans: (d) Q40. Two thin lenses of focal lengths f1 and f2 are in contact and coaxial. The power of the combination is: [2008]

(a)

(b)

(c)

(d) Ans: (d) Q41. A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the Sun is 1.39 ×109 m and its mean distance from the earth is 1.5 × 1011 m. What is the diameter of the Sun’s image on the paper? [2008] (a) 9.2 × 10–4 m (b) 6.5 × 10–4 m (c) 6.5 × 10–5 m (d) 12.4 × 10–4 m Ans: (a)

MockTime.com Q42. A ray of light travelling in a transparent medium of refractive index u, falls on a surface separating the medium from air at an angle of incidence ofFor which of the following value of u the ray can undergo total internal reflection? [2010] (a) u= 1.33 (b) u= 1.40 (c) u= 1.50 (d) u= 1.25 Ans: (c) Q43. Which of the following is not due to total internal reflection? [2011] (a) Working of optical fibre (b) Difference between apparent and real depth of pond (c) Mirage on hot summer days (d) Brilliance of diamond Ans: (b) Q44. A biconvex lens has a radius of curvature of magnitude 20 cm. Which one of the following options best describe the image formed of an object of height 2 cm placed 30 cm from the lens? (a) Virtual, upright, height = 1 cm [2011] (b) Virtual, upright, height =0.5 cm (c) Real, inverted, height = 4 cm (d) Real, inverted, height = 1cm Ans: (c) Q45. A thin prism of angle 15° made of glass of refractive index µ1 = 1.5 is combined with another prism of glass of refractive index µ2 = 1.75. The combination of the prism produces dispersion without deviation. The angle of the second prism should be [2011M] (a) 7° (b) 10° (c) 12° (d) 5° Ans: (b) Q46. A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is [2011M] (a) – 10 cm (b) 20 cm (c) –30 cm (d) 5 cm Ans: (c) Q47. When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index. [2012] (a) equal to that of glass (b) less then one (c) greater than that of glass

(d) less then that of glass Ans: (a) Q48. A ray of light is incident at an angle of incidence, i, on one face of prism of angle A(assumed to be small) and emerges normally from the opposite face. If the refractive index of the prism is P, the angle of incidence is nearly equal to: [2012]

(a)

(b)

(c)

(d) Ans: (a) Q49. The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are: 2012 (a) 10 cm, 10 cm (b) 15 cm, 5 cm (c) 18 cm, 2 cm (d) 11 cm, 9 cm Ans: (c)

Q50. For the angle of minimum deviation of a prism to be equal to its refracting angle, the prism must be made of a material whose refractive index: (a) lies between root2 and 1 (b) lies between 2 and root2 (c) is less than 1 (d) is greater than 2 Ans: (b) Q51. A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such a way that its end closer to the pole is 20 cm away from the mirror. The length of the image is: [2012M] (a) 10 cm (b) 15 cm (c) 2.5 cm (d) 5 cm Ans: (d)

MockTime.com Q52. The condition under which a microwave oven heats up a food item containing water molecules most efficiently is 2003 (a) the frequency of the microwaves has no relation with natural frequency of water molecules. (b) microwaves are heat waves, so always produce heating. (c) infra-red waves produce heating in a microwave oven. (d) the frequency of the microwaves must match the resonant frequency of the water molecules. Ans: (d) Q53. A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices u1 and u2 and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is [NEET2013]

(a)

(b)

(c)

(d) Ans: (b) Q54. For a normal eye, the cornea of eye provides a converging power of 40Dand the least converging power of the eye lens behind the cornea is 20D. Using this information, the distance between the retina and the eye lens of the eye can be estimated to be [NEET2013] (a) 2.5 cm (b) 1.67 cm (c) 1.5 cm (d) 5 cm Ans: (a) Q55. Two plane mirrors are inclined at 70°. A ray incident on one mirror at angle θ after reflection falls on second mirror and is reflected from there parallel to first mirror. The value of θ is [NEETKar. 2013] (a) 50° (b) 45° (c) 30° (d) 55° Ans: (a) Q56. The reddish appearance of the sun at sunrise and sunset is due to [NEETKar. 2013] (a) the colour of the sky (b) the scattering of light

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