What is the Doppler Effect

The Doppler Effect is one of the most fascinating effects that is observed in the propagation of waves. It deals with the apparent distortion that happens in the propagation of waves when the source or the observer of the wave are in relative motion. 

According to the Doppler effect, the wave’s frequency has an apparent change when it is received by an observer when there is a nonzero relative motion between the source of the wave and the observer.

The basic mechanics of the Doppler effect imply that the pitch or the frequency of the wave changes as the distance between the observer and the source changes. When the distance that separates the observer and the source increases, the pitch or the frequency decreases. When the distance separating the observer and the source decreases, the pitch or frequency increases.

The reason behind the Doppler Effect

When a body produces a sound, it propagates in the form of waves across the medium. These waves are like concentric circles that keep going away from the source. When the source is stationary, you can imagine the sound waves propagating through a medium like the ripples in a lake when you drop a stone in it. 

The concentric circles of sound waves travel at an equal speed, and the distance between each sound wave is called the wavelength. The wavelength remains constant when the source is constant. This means that to a stationary observer, the sound will be heard at the exact original frequency.

Doppler effect is observed when either the source or the observer or both are in relative motion to each other.

Movement of the source

As discussed, a stationary source will emit sound waves in the form of concentric circles, and the wavelength between each consecutive sound wave will be constant. When the source moves, the wavelength between these sound waves changes.

A source in motion, say towards an observer, will still produce sound waves just like when it was stationary. But now, the crest of each new consecutive wave will be at a position that is closer to the position of the observer. This means that the crest of the waves between the observer and the source will get bundled together, causing a higher frequency to be heard. 

Similarly, when the source is moving away, each successive crest of the sound wave is farther away from the observer’s position. This means that the crest of the waves between the source and the observer will be spread out, causing the apparent frequency to be lower than the original one.

Movement of the observer

When a stationary source is producing sound waves, they spread out from the source in the form of concentric circles. Assume that an observer is at a certain distance from this source. As long as the observer is stationary, they will hear only the original frequency since the wavelength will be constant between the consecutive sound waves.

If an observer moves towards the source, they constantly decrease the distance between them and the source. This means that they will hear the next soundwave at a time earlier than expected. This will happen for multiple soundwaves in a short period. Therefore, the observer will hear the sound waves in a bundled-up form. This causes the observer to hear the sound waves at a higher frequency than the house’s original frequency.

Conversely, when an observer is going away from the source, the distance between the observer and the source is constantly increasing. This means that the observer will hear the sound waves with an increasing delay between each consecutive sound wave. The effect this produces is that the sound waves will be heard more spread out than they originally are. This causes the observer to hear the sound waves at a lower apparent frequency than the original. 

The formula for Doppler effect

The Doppler effect is primarily observed in two types of waves, sound waves, and light waves. In the case of sound waves, it involves the change in the pitch with which the sound is heard. But in the case of light waves, the frequency changes; therefore, the wavelength of the light changes. This means that the colour of the light also changes.

The formulas for Doppler effect in all the possible causes are;

When the source is moving towards the observer but the observer is stationary;

In all these formulas;

f is the frequency apparent to the observer

fo is the frequency of the wave at the source

v is the velocity of propagation of the wave

vo is the observer’s velocity

v is the velocity of the wave in the medium

Conclusion

When a body produces a sound, it propagates in the form of waves across the medium. These waves are like concentric circles that keep going away from the source. When the source is stationary, you can imagine the sound waves propagating through a medium like the ripples in a lake when you drop a stone in it. 

The concentric circles of sound waves travel at an equal speed, and the distance between each sound wave is called the wavelength. The wavelength remains constant when the source is constant. This means that to a stationary observer, the sound will be heard at the exact original frequency.

Doppler effect can be observed when either the source of the wave or the observer or both are in relative motion to each other.