9.18:
Answer the following questions:
(i) You have learnt that plane and convex mirrors produce virtual images of objects. Can they produce real images under some circumstances? Explain.
(ii) A virtual image, we always say, cannot be caught on a screen. Yet when we see a virtual image, we are obviously bringing it onto the screen (i.e., the retina) of our eye. Is there a contradiction?
(iii) A diver underwater, 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?
(iv) Does the apparent depth of a tank of water change if viewed obliquely? If so, does the apparent depth increase or decrease?
(v) The refractive index of diamond is much greater than that of ordinary glass. Is this fact of some use to a diamond cutter.
Hint: Converging rays form a real image.
Step: Find the image formed by the converging rays on a plane mirror and convex mirror.
If the object is virtual and the light rays converging at a point behind a plane mirror (or a convex mirror) are reflected back to a point on a screen placed in front of the mirror, then a real image will be formed.
(b)
Hint: Recall the properties of a convex lens.
Step: Determine the nature of the image formed by the convex lens of the eye.
The convex lens of the eye converges these diverging rays at the retina and a real image is formed at the retina while the virtual image serves as an object for the lens to produce a real image.
(c)
Hint: light rays bend towards the normal when they travel from a rarer medium to a denser medium.
Step: Find the height of the fisherman with respect to the driver.
The diver is viewing the fisherman from a denser medium. As the light rays bend towards the normal when they travel from a rarer medium to a denser medium. So the rays will appear to the diver coming from a point higher than the height of the fisherman. So the fisherman appears taller to the diver.
(d)
Hint: light bends away from the normal when it travels from a denser medium to a rarer medium.
Step: Find the apparent depth of a tank of water.
The apparent depth of a tank of water changes when viewed obliquely. This is because the light bends away from the normal when it travels from a denser medium to a rarer medium. The apparent depth of the tank, when viewed obliquely, is less than its actual depth.
(e)
Hint: Refractive index of diamond is much greater.
Step: Find the angle of incidence for which TIR is possible.
The refractive index of a diamond (2.42) is more than that of ordinary glass (1.5). The critical angle for diamond is less than that for glass. A diamond cutter uses a large angle of incidence to ensure that the light entering the diamond is totally reflected from its face. This is the reason for the sparkling effect of a diamond.
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