question_answer 17)

Answer the following questions ; (a) You have learned that plane and convexmirrors produce virtual images of objects. Canthey produce real images under somecircumstances ? Explain. (a)A virtual image, we always say, cannot be caught on a screen. Yet when we 'see' a virtualimage, we are obviously bringing it on to the 'screen' (i.e., the retina) of our eye. Is there acontradiction ? (c) A diver under water, looks obliquely at afisherman standing on the bank of a lake, wouldthe fisherman look taller or shorter to thediver than what he actually is ? (d) Does the apparent depth of a tank of waterchange if viewed obliquely ? If so, does theapparent depth increase or decrease ? (e) The refractive index of diamond is much greater than that of ordinary glass. Is this factof some use to a diamond cutter ?  

Answer:

(a) Yes. They can produce real images if theobject is a virtual object. (b) There is no contradiction in this case.  The virtual image of the object acts as anobject for the convex lens of our eye andthe lens of our eye make a real image of this object on the retina. (c) Let AB be the fisherman standing on thebank of the lake. The rays of light from thehead of the fisherman bends towards thenormal on refraction at the interfaceseparating water and air. The refracted raysappear to come from point B' instead of pointB for the fish'. Thus, for a diver the height ofthe fisherman is AB' which is greater thanhis actual height AB. (d) The apparent depth of a pond of waterdecreases when viewed obliquely. This is dueto the refraction of light from the surface ofwater. (e) The refractive index of diamond is 2.5which gives, the critical angle as 24°. Thefaces of the diamond are cut in such a waythat whenever light falls on any of the faces,the angle of incidence is greater than thecritical angle i.e. 24°. So when light falls onthe diamond, it Suffers repeated internalreflections. The light which finally emergesout from few places in certain directionsmakes the diamond sparkling.