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Answer the following questions: (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. Would the fisherman look taller or shorter to the diver than what he actually is? (d) Does the apparent depth of a tank of water change if viewed obliquely? If so, does the apparent depth increase or 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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Solution

a)

Yes, plane and convex mirrors can also produce real images. It can be possible if the object is virtual and the rays from the object converge at a point behind the plane or convex mirror and get reflected to a point which is in front of a mirror on a screen.

b)

The condition for the image formed to be virtual is that, the reflected or the refracted rays from it must be divergent. The convex lens of the eye then converge these divergent rays on the retina. Thus, the virtual image formed on the retina now serves as an object for the lens to produce a real image. Therefore; there is no contradiction that a virtual image cannot be caught on the screen.

c)

When the diver looks at the fisherman then, the light rays travel from water to air, that is, from denser to rarer medium. As the refracted light rays travel from denser to rarer medium it moves away from the normal. Due to this effect, the height of the fisherman will appear taller from normal.

d)

The apparent depth is directly dependent on the angle of refraction, and as the angle of refraction decreases when viewed obliquely, the apparent depth also decreases.

e)

The refractive index of ordinary glass is less than that of diamond. Hence, the critical angle for diamond is less than that of glass.

The diamond cutter ensures that there is a large angle of incidence for the light entering the diamond. The light rays then will get total internal reflection within the faces of the diamond and gives the required sparkling effect.


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