Q.

A bird B in air is diving vertically downwards over a water tank (made up of  transparent glass) with speed 5 cm/s. Base of tank is silvered. A fish ‘F’ in the tank is  rising vertically upwards along the same line with speed 2 cm/s. Water level in the  tank is decreasing at the rate of 2 cm/s. μwater=43, At that instant speed of  image/images of fish as seen by the bird is/are (in cm/s)
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a

8

b

6

c

3

d

4

answer is C, D.

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Detailed Solution

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D) Direct image distance of fish wrt bird is
XFB=x1+x2 μ 
XFB=dx1dt+1μdx2 dt 
= - (5 – 2)+ 34 (-2 -2)
-3 + (- 3) = - 6 cm/s
C) mirror image distance of fish wrt bird is
XFB=x1+h μ+hx2μ 
VFB=dx1dt+2 μdhdt1μdx2dt 
=(52)+234(2)34(4) 
=33+3=3  cm/s 

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A bird B in air is diving vertically downwards over a water tank (made up of  transparent glass) with speed 5 cm/s. Base of tank is silvered. A fish ‘F’ in the tank is  rising vertically upwards along the same line with speed 2 cm/s. Water level in the  tank is decreasing at the rate of 2 cm/s. μwater=43, At that instant speed of  image/images of fish as seen by the bird is/are (in cm/s)