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GATE 2022 IN – Question 39

Communication and Optical Instrumentation · Interferometry and fiber-optic sensing · 2 marks · Multiple choice

A car moves collinearly with a transceiver’s laser beam. A pulse emitted at $t=0$ returns after reflection from the car 100 ns later. A second pulse emitted at $t=0.1$ s returns 90 ns later. Given light speed $3\times10^8$ m/s, the average car speed is ______.

  1. 54 km/h, towards the transceiver
  2. 108 km/h, towards the transceiver
  3. 54 km/h, away from the transceiver
  4. 108 km/h, away from the transceiver

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Correct answer: (A) 54 km/h, towards the transceiver

Explanation

Round-trip delay gives range ct/2: initially 15 m and later 13.5 m. The car approaches by 1.5 m in 0.1 s, speed 15 m/s=54 km/h.

Therefore **54 km/h towards the transceiver, A**.