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GATE 2021 IN – Question 44

Communication and Optical Instrumentation · Optical sources and detectors · 2 marks · Multiple choice

A laser pulse travels normally from ground level to the bottom of a concrete water tank and returns to a detector at ground level in 100 ns. Water begins 2 m below ground. Given water index 1.3 and air light speed $3\times10^8$ m/s, the total tank depth $x$ is ______ m.

Diagram for GATE 2021 IN question 44
  1. 9
  2. 10
  3. 11
  4. 12

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Correct answer: (D) 12

Explanation

The one-way equivalent air path is ct/2=3×10⁸·100×10⁻⁹/2=15 m. With 2 m of air and x−2 m of water, 2+1.3(x−2)=15. Solving gives x=2+13/1.3=**12 m, D**, including the air layer.