Question

An enemy plane is at a distance of $300$km from a radar. In how much time the radar will be able to detect the plane? Take velocity of radio-waves as $3\times {10}^{8}m{s}^{-1}$.

Answer 1

The radio waves, at first, reach to the plane. Then they get reflected by the plane and so the reflected radio waves reach the radar again where they get detected. So, radio waves have to travel twice the distance between the plane and the radar.

Distance between the plane and radar $d=300km$

Distance traveled by radio waves $D=2d=2\times 300=600km$

$\therefore$ $D=600\times {10}^{3}m$

Speed of radio waves $v=3\times {10}^{8}m/s$

So, time taken by radar to detect the radio waves $t=\frac{D}{v}$

$\therefore$ $t=\frac{600\times {10}^3}{3\times {10}^8}=2\times {10}^{-3}s$