# An object is placed at some distance from a radio station. If the interval between transmission and reception of pulses is $2.66 \times 10^{-2}\: sec$, then the distance is:

$\begin {array} {1 1} (a)\;4000\: km & \quad (b)\;2000\: km \\ (c)\;3000\: km & \quad (d)\;2500\: km \end {array}$

Time difference between transmission and reception of pulses $= T = 2.66 \times 10^{-2}s$
So, time requires for pulse to travel from station to the object $= t =\large\frac{T}{2}$$= \large\frac{2.66 \times 10^{-2}}{ 2}$
$= 1.33 \times 10^{-2}s$
So, distance $= t \times$ speed of radio wave in air,
since radio waves are electromagnetic waves its speed is the speed of light
$= 1.33 \times10^{-2}s \times3 \times10^8m/s = 3990 \: km \sim 4000 \: km$
Ans : (a)

edited Oct 3, 2014