Class 12 Physics Chapter 9: Ray Optics and Optical Instruments MCQs with answers
10 multiple-choice questions on Ray Optics and Optical Instruments, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.
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Answer: (c) u negative, f positive
Distances are measured from the pole, positive along the incident light. The object is on the side the light comes from, so u is negative; the focus of a convex mirror lies behind it, along the incident light, so f is positive.
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Answer: (a) 3.0 cm
In normal adjustment the tube length is fo + fe = 93 cm and the magnifying power is fo/fe = 30, so fo = 30 fe and 31 fe = 93 cm, fe = 3.0 cm (fo = 90 cm). 3.1 cm comes from dividing the length by 30, forgetting that the length also contains fe; 90 cm is the objective.
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Answer: (d) retraces its path
A ray directed towards the centre of curvature meets the mirror along the normal (the radius), so the angle of incidence is zero and the ray is reflected back along its own path.
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Answer: (b) −12 cm
Object and real image are both in front of the mirror, so u = −18 cm and v = −36 cm. 1/f = 1/v + 1/u = −1/36 − 1/18 = −3/36, f = −12 cm. +12 cm drops the sign of a concave mirror; −27 cm is the average of the two distances, not a focal length.
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Answer: (b) 30 cm
The image of a convex mirror is erect, so m = +0.4. With m = −v/u and 1/v + 1/u = 1/f, m = f/(f − u), so u = f − f/m = 20 − 50 = −30 cm: the object is 30 cm in front of the mirror (and v = +12 cm). 12 cm is the image distance; 50 cm is f/m alone; 8.0 cm is f × m.
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Answer: (d) 5 cm, away from the mirror
f = −10 cm. At u = −30 cm, 1/v = −1/10 + 1/30 = −1/15, v = −15 cm. At u = −20 cm, 1/v = −1/10 + 1/20 = −1/20, v = −20 cm. The image moves from 15 cm to 20 cm in front of the mirror: 5 cm away from it, as the object moves 10 cm towards it.
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Answer: (a) the wavelength of the light
n21 is a property of the pair of media and depends on the wavelength of light, not on the angle of incidence. Because it differs for different colours, white light splits into its colours (dispersion).
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Answer: (c) 2.4 × 10⁸ m s⁻¹
The refractive index is the ratio of the speeds of light, n = c/v, so v = 3.0 × 10⁸/1.25 = 2.4 × 10⁸ m s⁻¹. 3.75 × 10⁸ (multiplying) would exceed c, which no speed can.
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Answer: (d) 9/8
n(glass, water) = n(glass, air) × n(air, water) = (3/2) × (3/4) = 9/8. 8/9 is the index of water with respect to glass.
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Answer: (b) parallel to the incident ray but shifted sideways
Refraction at the two parallel faces gives r2 = i1, so the emergent ray is parallel to the incident ray: there is no deviation, only a lateral shift.