Waves

JEE Main level, single correct, medium

118 of 142

Question 118 of 142

A radar sends an electromagnetic signal of electric field (E0)=2.25Vm−1\left({E}_{0}\right)=2.25V{m}^{-1} and magnetic field (B0)=1.5×10−8T\left({B}_{0}\right)=1.5\times {10}^{-8}T which strikes a target on line of sight at a distance of 3km3km in a medium. After that, a part of signal (echo) reflects back towards the radar with same velocity and by same path. If the signal was transmitted at time t=0t=0 from radar, then after how much time echo will reach to the radar?

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Answer: (B) 4.0×10−5s4.0\times {10}^{-5}s

The speed of light is given by,
c=E0B0=2.251.5×10−8=1.5×108ms−1c=\dfrac{{E}_{0}}{{B}_{0}}=\dfrac{2.25}{1.5\times {10}^{-8}}=1.5\times {10}^{8}{ms}^{-1}
Therefore, time taken for the echo will be,
t=2dc=6×1031.5×108=4×10−5st=\dfrac{2d}{c}=\dfrac{6\times {10}^{3}}{1.5\times {10}^{8}}=4\times {10}^{-5}s

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