The Doppler effect is applicable for

$A$ rocket is moving towards the moon with a speed $v$. The astronaut in the rocket sends signals of frequency $f$ towards the moon and receives them back after reflection from the moon. What will be the frequency of the signal received by the astronaut? (Assume $v << c$)

$A$ 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 ....... $\mathring{A}$.

$A$ light source approaches the observer with velocity $0.8 c$. The Doppler shift for the light of wavelength $5500 \ \mathring{A}$ is.....$\mathring{A}$.

Assuming that the universe is expanding,if the spectrum of light coming from a star which is moving away from the Earth is tested,then in the wavelength of light:

$A$ star is moving towards the earth with a speed of $4.5 \times 10^6 \ m/s$. If the true wavelength of a certain line in the spectrum received from the star is $5890 \ \mathring A$,its apparent wavelength will be about........$\mathring A$ $[c = 3 \times 10^8 \ m/s]$