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# Question 18: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments

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Question 18: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments

(a) You have learnt that plane and convex mirrors produce virtual images of objects. Can they produce real images under some circumstances? Explain.
(b) A virtual image, we always say, cannot be caught on a screen. Yet when we’see’a virtual image, we are obviously bringing it on to the ‘screen’ (i.e., the retina) of our eye. Is there a contradiction?
(c) A diver under water, looks obliquely at a fisherman standing on the bank of a lake.
(d) Does the apparent depth of a tank of water change if viewed obliquely? if so, does the apparent depth increase of decrease?
(e) The refractive index of diamond is much greater than that of ordinary glass. is this fact of some use to a diamond cutter?

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1. Solution:
(a) In this situation when rays are convergent behind the mirror, both plane mirror and convex mirror can form real images of virtual objects.

(b) Here, the retina is working as a screen, where the rays are converging, but this screen is not at the position of formed virtual image, in fact the reflected divergent rays are converged by the eye lens at retina. Thus, there is no contradiction.

(c) An observer in denser medium will observe the fisherman taller than actual height, due to refraction from rare to denser medium.

(d) Apparent depth decreases if viewed obliquely as compared to when observed near normally.

(e) As

hence, sin

refractive

index of diamond is much greater than that of ordinary glass, hence critical angle C for diamond is much smaller (24°) as compared to that of glass (42°).
A skilled diamond cutter thus can take the advantage of such large range of angle of incidence available for total internal reflection 24° to 90°. The diamond can be cut with so many faces, to ensure that light entering the diamond does multiple total internal reflections before coming out. This behavior produce brilliance i.e., sparkling effect in the diamond.

Check the complete chapter with solutions.

NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments