An observer is moving with half the speed of light towards a stationary microwave source emitting waves at frequency $10\; GHz$. What is the frequency of the microwave measured by the observer ? (speed of light $=3 \times 10^8 ms^{−1}$ )

Question

$\begin{array}{1 1} (1)\;10.1\; GHz \\ (2)\; 12.1\; GHz \\(3)\; 17.3\; GHz \\ (4) 15.3 GHz \end{array} $