(1) Determination of velocity of a star or a galaxy:- When the light emitted by a star is examined, the spectrum is found to consist of many colours (i.e., spectral lines). If the star approaches the earth, the shift of spectral lines of the spectrum takes place towards the violet end of the spectrum. This is called violet or blue shift and indicates the decrease in wavelength or increase in frequency.

If the star recedes away from the earth, the shift of spectral lines takes place towards the red end of the spectrum. The shift is called red shift and indicates the increase in wavelength or decrease in frequency.

Suppose as star (i.e., source) of light is moving away from the earth (i.e., observer) with velocity v, the apparent frequency of the light waves emitted by the star is

v' = ({c}/{c + v})v,    ....(i)

where c is the velocity of light

Now v = c/λ and v' = c/λ'

Hence, c/λ' = {c}/{c + v} x c/λ

or, λ'/λ = {c + v}/{c} = 1 + v/c

or, λ'/λ - 1 = v/c or {λ' - λ}/{λ} = v/c

or, Δλ/λ = v/c, where Δλ = λ' = λ

or, v = c x Δλ/λ'

where c = 3 x 10⁸ ms^-1    ...(ii)

Knowing the change in wavelength (Δλ), we can calculate the velocity (v) of the star.

(2) Radar (Radio Detection and Ranging):- It consists of a radio transmitter which sends pulses of electromagnetic radiation and a radio receiver, which receives the echoes due to the reflection of these radiation from the objects in their way.

According to Doppler's effect, if the object (say aircraft) is approaching the radar, then the reflected waves shall have a higher frequency (or lower wavelength) than the transmitted waves. If the object is receding away from the radar, then the reflected waves will have a lower frequency (or higher wavelength) than the transmitted waves.

(3) Sonar (Sound Navigation and Ranging):- In case of sonar, the waves, (i.e., ultrasonic waves) are sent from the ship towards the bed of the sea. If the submarine is approaching the ship, the reflected waves from the submarine will have higher frequency than the transmitted waves and vice-versa.

(4) Doppler's effect is also used to determine the speed of rotation of the sun.It has been calculated as 2 km s^-1.