Doppler’s Effect: Introduction, Definition, and History

The study of waves is an important part of the physics syllabus. There can be two types of waves – electromagnetic and matter waves, with the frequency of waves defining their nature. Doppler’s Effect was devised by the Austrian physicist named Christian Doppler.

Let us understand Doppler’s Effect, its brief history, and its formula. We cover the basic applications of this Effect and quick definitions to help students go through the main concepts. We will also have a quick look at the immediate understanding of waves, basic advantages of the Doppler’s Effect, the speed of sound in air, and this Effect in both light and sound.

What is Doppler’s Effect?

It is the phenomenon observed during the movement of sound waves relative to the observer. Doppler’s Effect in physics is defined as the increase or decrease in the frequency of sound waves or light waves when the source of waves moves away from or towards the observer. 

History of Doppler’s Effect

• Doppler, a popular Austrian physicist, proposed this Effect in his treatise “On the Coloured Light of the Binary Stars and Other Stars of the Heavens” in 1842.

• Buys Ballot tested Doppler’s hypothesis on sound waves in 1845.

• Hippolyte Fizeau discovered Doppler’s Effect on electromagnetic waves in 1848. The same year, John Scott Russell in Britain conducted an experimental study on the Doppler’s Effect. 

Doppler Effect’s Formula

The formula for the Doppler’s Effect can be given as follows:

fo = (v + vo) / (v + vs) * fs, where,

fo is observer frequency of sound

fs is the actual frequency of sound waves

vo is the velocity of the observer

vs is the velocity of the source

v is the speed of sound waves

Doppler’s Effect on Sound

Before jumping to the Doppler shift in sound, it is essential to know the speed of sound in the air. It is estimated to be 331.29 metres or 1,086.9 feet per second for dry air at a temperature of 0 °Celsius. Hence, the Doppler’s Effect in sound defines the sudden increase or decrease of frequency when the source of sound and observer is moving towards or away from each other.

Doppler’s Effect in Light

Doppler’s Effect on light can be explained as the sudden change in the frequency of the light observed by the observer due to the relative motion between the observer and the source of light. Light needs no medium to travel, and Doppler’s Effect for light travelling in a vacuum depends on the relative speed of the source and the observer.

The redshift and blue shifts observed by astronomers are due to Doppler’s shift for light. The shift of light towards the red end of the visible light spectrum when the light source moves away from the observer is called the redshift. The shift of light towards the high-frequency end of the visible light spectrum when the light source moves towards the observer is called the blue shift.

Advantages of Doppler’s Effect

A few of the key applications and benefits of Doppler’s Effect in light and sound are:

1. Measurement of the velocity profile

2. Audio and sirens

3. Development biology, blood flow measurement, and medical imaging

4. Radar

5. Satellite communication, vibration measurement, etc.

6. Astronomy

Limitations of Doppler’s Effect

Doppler’s Effect is applicable only if the observer and the source lie along a straight line. Further, it is applicable only if the velocity of the source of the sound and the observer is less than the velocity of the sound.

Conclusion

Doppler’s Effect, which helps guide the variation in sound when the prime sound source is moving, is quite simple to understand. Its history dates back to its observation by the Austrian physicist Christian Doppler in 1842. It is easy to understand the speed of sound in air and the Doppler Effect’s formula.

Doppler’s Effect in sound defines the sudden increase or decrease of frequency when the source of sound and observer is moving towards or away from each other. The redshift and blue shifts observed by astronomers are due to Doppler’s shift for light.