Assertion: For best contrast between maxima and minima in the interference pattern of young's double slit experiment, the intensity of light emerging out of the two slits should be equal.

Reason: The intensity of interference pattern is proportional to square of amplitude.

Assertion: In Young's double slit experiment, the fringes become indistinct if one of the slits is covered with partially opaque paper.

Reason: The partially opaque paper decreases the wavelength of light.

Assertion: The unpolarized light and polarised light can be distinguished from each other by using Polaroid.

Reason: A Polaroid is capable of producing plane polarised beams of light.

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