Question

A fish in an aquarium approaches the left wall at 2.5 m/s observes a fly approaching it at 8 m/s. If the refractive index of water is 4/3. Find the actual velocity of fly.

• A. 8 m/s
• B. 10.67 m/s
• C. 13.17 m/s
• D. 16 m/s

Answer 1

Answer: (C)

13.17 m/s

Answer 2

The fish’s observed closing rate is “reduced” by the factor 1/n. Multiplying 8 m/s by 4/3 gives the true closing speed between the fly and fish (32/3 m/s). Since the fish is moving at 2.5 m/s, add that to get the fly’s speed:

$v_{fly}=\frac{32}{3} +2.5=\frac{39.5}{3}\approx 13.17\,\text{m/s}.$