A submarine is observed to rise from a real depth of 80m to 60m in water. Calculate the change in apparent depth. [Refraction index of water 4/3]

A. 45 m
B. 80 m
C. 15 m
D. 60 m

Correct Answer:

Option C – 15 m

Explanation

Refraction index (n)= (Real depth)/App. depth
For 80 m Real depth: (4)/3 = (80)/App. depth
App. dept = (80 x 3)/4 = 60 m
For 60 m real depth; (4)/3 = (60)/ App. depth
App. depth = (60 x 3)/4 = 45 m
∴change in App. depth = 60 -45 = 15 m