# Doppler effect or Doppler shift

The apparent change in frequency/wavelength of a wave caused by a difference in velocity between the source of the wave and the observer is named the Doppler effect or Doppler Shift.

## Why the Doppler Shift Happens | Doppler effect causes

The Doppler shift occurs because a wave is created by a series of pulses at regular intervals, and the wave moves at a particular speed.

If the source is approaching, each pulse arrives sooner than it would have if the source had been stationary. Because the frequency is the number of pulses that arrive in one second, the moving source results in an increase in the frequency observed by the receiver.

Similarly, if the source is moving away from the observer, each pulse arrives later, and the observed frequency is lower.

## The change in frequency due to the Doppler effect (equation)

The change in frequency due to the Doppler effect is given by the equation:

**The rule for adding or subtracting velocities is:**

– The receiver’s (your) velocity is in the numerator. If you are moving toward the sound, this makes the pulses arrive sooner, which makes the frequency higher. So if you are moving toward the sound, add your velocity. If you are moving away from the sound, subtract your velocity.

– The source’s velocity is in the denominator. If the source is moving toward you, this makes the frequency higher, which means the denominator needs to be smaller. This means that if the source is moving toward you, subtract its velocity. If the source is moving away from you, add its velocity.