- Doppler Effect states that there is a change in the frequency of sound because of the relative motion between the observer or the listener and the source of the object.
- In simpler words when the source and the listener are in a relative motion there is a change in the frequency of sound and this effect of a change in frequency is known as the Doppler's Effect of sound.

- When the objects move apart the frequency decreases.
- When the objects move towards each other the frequency increases.
- The change in the frequency of the sound is given by the formula:

**n = n' (V±Vo / V±Vs)**

Where, n = actual frequency of sound

n' = Apparent Frequency

v = Speed of sound

Vo = Spped of the observer

Vs = Speed of the source

## Conditions

**1. When the observer is at rest i.e. Vo = 0**

In this case, the formula becomes:

n = n' (V / V±Vs)

a) If the source is moving away from the observer then Vs will be positive as

**n**decreases with increase in Distance.
b)If the source is coming towards the observer then Vs will be negative.

**2.When the source is at rest i.e. Vs = 0**

n = n' (V±Vo / V)

a)If the observer is moving away from the source then Vo will be negative.

b)If the source is coming towards the observer then Vo will be positive.

**3.Both are moving**

In this case, the formula becomes:

n = n' (V±Vo / V±Vs)

a)If both are moving away then Vo will be negative and Vs will be positive.

b)If both are moving closer then Vo will be positive and Vs will be negative.