# Doppler Effect

## Doppler Effect

Doppler Effect refers to the change in wave frequency during the relative motion between a wave source and its observer. It was discovered by Christian Johann Doppler who described it as the process of increase or decrease of starlight that depends on the relative movement of the star.

What is Doppler Effect?

The apparent change in frequency of sound wave due to the relative motion of source of sound of sound and observer is called Doppler’s Effect.

Example: You hear the high pitch of the siren of approaching ambulance and you notice dropping of pitch suddenly as ambulance passes you which is Doppler Effect. This phenomenon was first derived by Australian Scientist Doppler. So it is Doppler’s Effect.

Formula to find the Doppler Effect:

Doppler Effect formula is used to find the relationship between observed frequency and the emitted frequency where the speed of source and receiver are lower than the velocity of the waves in the medium.

$$f=\left( \frac{c\pm {{v}_{r}}}{c\mp {{v}_{s}}} \right){{f}_{0}}$$;

Where,

c = Velocity of waves in the medium,

vr = Velocity of the receiver relative to the medium,

vs = Velocity of the source relative to the medium,

f = Observed frequency,

f₀ = Emitted frequency.

Applications of Doppler Effect:

Astronomy: In astronomy, Doppler Effect for electromagnetic waves of light results in either red shift or blue shift. With the help of Doppler Effect and radial velocity, one can measure the speed at which stars or galaxies are receding or approaching from us.

Velocity profile measurement: Ultrasonic Doppler Velocimetry is used to measure the real-time completion velocity profile of any liquids containing suspended particles like dust, emulsions and gas bubbles. The flow can be pulsating, laminar or turbulent, oscillating or stationary.

Sirens: The principle behind siren is that it starts out at a higher pitch than its stationary pitch when it slides down from the observer and again when it recedes from the observer it continues from the lower pitch than its stationary pitch. It is used in emergency vehicles.