# Doppler Effect For Moving Observer: What, How, Examples & FAQs

The Doppler effect is the variation in the frequency of light and sound waves. In this article, we shall learn the causes of the Doppler effect for moving observers.

The Doppler effect for moving observers increases the intensity and frequency of waves if the distance between the observer and the source is reduced and decreases the frequency of the waves if the observer is moving away from the source. It occurs due to the change in the time required for waves to reach the ears.

The Doppler effect is used for sound effects in the orchestra, to estimate the velocity of heavenly bodies, etc. We shall further discuss the fact why the Doppler effect is seen only when the observers are moving and how the effect varies if the observer is moving towards and away from the source.

## Why does the doppler effect occur when an observer is moving?

The Doppler effect is experienced only when the observer is moving. Let us understand the reason behind this effect and understand the Doppler effect in detail.

The Doppler effect occurs only when the observer is moving because there is a change in the wave frequency due to the lag in time for a wave to reach the observer and the frequency is inversely related to the time. It is seen if the observer is moving away or towards the source, and the source is in motion.

## Doppler Effect for the Observer Moving Towards Source

While the observer is moving toward the source, the frequency of the wave is increased. Let us discuss about the Doppler effect for the observer moving toward the source.

The Doppler effect intensifies the wave frequency as the distance between the stationary source and the observer decreases. The formula to calculate the wave frequency is, fD = (v + vo)f/ v, here, fD is the doppler frequency, f is the actual frequency, v is the actual velocity and vo is the observer velocity.

The given formula indicates that the Doppler frequency for the observer moving towards the stationary source is proportional to the ratio of the sum of the velocities of the observer and the wave velocity in the medium to the velocity of the wave in that medium.

## Doppler Effect for the Observer Moving Away from Source

The observer moving away from the source will increase the distance between the two. Let us see how the expanding distance will generate the Doppler effect for the observer.

The Doppler effect reduces the wave frequency as the distance between the observer and source increases. The expression for the wave frequency in this scenario is given as fD = (v – vo)f/ v, here, fD is the Doppler frequency, f is the actual frequency, v is the wave velocity in medium and vo is the observer velocity.

The Doppler frequency for the observer moving away from the stationary source of waves is proportional to the ratio of the difference between the velocities of the observer and the wave velocity in the medium to the velocity of the wave in that medium.

## Moving Source and Moving Observer Doppler Effect

The Doppler effect is also observed if the observer as well as the source is in motion. Let us understand how the frequency will be affected due to the motion of both at a time.

The Doppler effect is seen for a moving source and the moving observer due to the variations in the distance between the two that fluctuate the Doppler frequency. It is expressed by the formula: fD = (v ± vo)f/ (v ± vs), here, v is the velocity in the medium, vo is the observer velocity and vs is the source velocity.

If the observer is moving away from the source then its velocity is taken as – vo and if it is moving towards the source then it is + – vo, and likewise for the source direction of motion with respect to the observer.

#### Conclusion

We can conclude from this article that the Doppler effect for moving observers is seen due to the frequent change in the time interval. As wave nodes reach the observer from the source is a varying distance. If the observer is moving away from the source then the time taken for a wave to reach the observer will increase.