Optics Question 123

Question: A fish rising vertically up towards the surface of water with speed 3 ms?1 observes a bird diving vertically down towards it with speed 9 ms?1. The actual velocity of bird is

Options:

A) 4.5 ms?1

B) 5. ms?1

C) 3.0 ms?1

D) 3.4 ms?1

Show Answer

Answer:

Correct Answer: A

Solution:

Here optical distance between fish and the bird is $ s=y’+\mu y $ Differentiating w.r.t t we get $ \frac{ds}{dt}=\frac{dy’}{dt}+\frac{\mu dy}{dt} $

Therefore $ 9=3+\frac{4}{3}\frac{dy}{dt} $

Therefore $ \frac{dy}{dt}=4.5\ m/\sec $



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