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 $
