KVPY 2019 SA Exam Previous Papers

Question : 66 of 80

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PHYSICS

A water–proof leser pointer of length 10 cm placed in a water tank rotates about a horizontal axis passing through its center of mass in a vertical plane as shown in the figure. The time period of rotation is 60s.
Assuming the water to be still andno reflections from the surface of the tank, the duration for which the light beam escapes the tank in one time period is close to (Refractive index of water = 1.33)

8.13 s
14.05 s
16.27 s
23.86 s

Validate

Solution:

Light will come out when the angle is less than critical angle 'C'

\sin c =1/1.33=3/4

⇒ c = 50° (approx)

ωt = 2c

t = {2×50/180×60}/{2π}= 16.27 \sec.

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