The Doppler effect is applicable for

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 $c = 3 \times 10^8 \ ms^{-1}$)

$A$ star emitting radiation at a wavelength of $5000 \; \mathring{A}$ is approaching Earth with a velocity of $1.5 \times 10^6 \; m/s$. The change in wavelength of the radiation as received on Earth is ....... $\mathring{A}$.

$A$ star emits light of $5500 \ \mathring{A}$ wavelength. If it appears blue to an observer on the Earth,it means:

Hubble showed that the universe as a whole is expanding and the distant stars are receding from us. The spectral line from a star,when compared with the corresponding line from a stationary source,will then show:

The sun is rotating about its own axis. The spectral lines emitted from the two ends of its equator,for an observer on the earth,will show