# Doppler Effect

When an ambulance crosses you with its siren blaring, you hear the pitch of the siren change: as it approaches, the siren’s pitch sounds higher than when it is moving away from you. This change is a common physical demonstration of the Doppler effect. But, have you taken the time to understand the phenomenon of the Doppler effect and its causes? If the answer is NO, then read on the article to understand clearly and answer any given questions on Doppler effect in exams.

## How is the Doppler Effect Defined?

Doppler effect is an important phenomenon that is useful in a variety of different scientific disciplines, including planetary science: Astronomers rely on the Doppler effect to detect planets outside of our solar system or exoplanets.  The Doppler effect or the Doppler shift describes the change in frequency of any kind of sound or light wave produced by a moving source with respect to an observer. Waves emitted by a source travelling towards an observer gets compressed. In contrast, waves emitted by a source travelling away from an observer get stretched out. We can define the Doppler effect as

Doppler Effect is an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.

Many mistake Doppler effect to be applicable only for sound waves. It should be noted that the Doppler effect doesn’t just apply to sound. It works with all types of waves including light.  Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble found that the light from distant galaxies was shifted toward lower frequencies, to the red end of the spectrum. This is known as a red Doppler shift or a red-shift. If the galaxies were moving toward Hubble, the light would have been blue-shifted. Doppler radars also help meteorologists learn about possible tornadoes

## Discovery of Doppler Effect

Doppler was the son of a stonemason, who went on to become a celebrated academic and scientist. After school young Christian studied astronomy and mathematics in Salzburg and Vienna, and at the age of 38 went on to work at the Prague Polytechnic in Czechoslovakia. Only a year later, he found fame by discovering that the observed frequency of light and sound waves is affected by the relative motion of the source and the detector (in other words their positions in relation to one another)–and this became known as the Doppler Effect. On 17 March 1853, at the age of only 49, Christian Doppler died from respiratory disease in Venice.

## Understanding Doppler Effect In Real Life

To understand the Doppler effect let us imagine the following scene. You are standing beside a road and a police car with its siren turned on, drives by you. What do you notice about the sound? The siren’s sound isn’t so loud when it is at a distance, which then reaches a maximum when it is just beside you, diminishing again as it crosses and moves away from you.

Consider another instance. Two people A and B are standing on the road as shown below in the picture.

Which person do you think hears the sound of the revving engine with a greater magnitude? You know that its person A. But why?

This is where we discuss the Doppler Effect or Doppler Shift. To highlight this effect you should understand the difference between the two situations given below.

Situation 1: How is the pattern of waves formed when you suddenly jump into a pond?

Situation 2: How is the pattern of waves formed when you are walking in a pond?

The image given below highlights the difference between wave patterns in both situations.

This difference is because the source of the waves in the second case moves. This is what the Doppler Effect is. It is named after the physicist Christian Doppler who proposed this in the 19th century. The Doppler effect is the change of frequency of a wave emitted as observed by an observer moving relative to the source. In this, the frequency received by the observer is higher during the approach, identical when the relative positions are the same, and keeps lowering on the recession of source. If both the source and observer are moving, the total Doppler Effect is calculated based on both these motions.

Let us say that light waves travel from a source to an observer. In this case, the wave travels the fixed distance across which the source and the observer are located. But there are cases when either of the two is moving, that is, the source is moving relative to the observer, or vice versa. It is in these scenarios that the Doppler effect comes into the picture.

In this video let’s see how relative motion between the source and the observer changes the frequency of sound waves giving rise to Doppler Effect.

## Doppler Effect Formula

In physics, where the speed of the receiver and the source relative to the medium are lower than the velocity of waves, the relationship between emitted frequency f0 and observed frequency f is given by:

 $f = (\frac{c\pm v_{r}}{c\pm v_{s}}) f_{0}$

where

• c is the velocity of waves in the medium
• vr is the velocity of the source relative to the medium
• vs is the velocity of the receiver relative to the medium

The frequency decreases if either is moving away from the other.

## Applications of Doppler Effect

Some Doppler effect applications are provided in the points mentioned below:

• Sirens
• Astronomy
• Medical Imaging
• Blood Flow Measurement
• Satellite Communication
• Vibration Measurement
• Developmental Biology
• Audio
• Velocity Profile Measurement

## Doppler Effect In Light

The Doppler effect also affects the light which is emitted by other bodies in space. If the body is “red shifted” the light waves are spread apart, and it is travelling away from us while if it is “blue shifted,” its light waves are compacted and it is coming towards us. The detailed explanation of the Doppler effect in light is given below.

### Red Shift and Blue Shift:

• When the light source moves away from the observer, the frequency received by the observer will be less than the frequency transmitted by the source. This causes a shift towards the red end of the visible light spectrum. Astronomers call it as the redshift.
• When the light source moves towards the observer, the frequency received by the observer will be greater than the frequency transmitted by the source. This causes a shift towards the high-frequency end of the visible light spectrum. Astronomers call it as the blue shift.

## Doppler Effect In Sound

For sound waves which propagate in a medium, the velocity of the source and the observer are relative to the medium in which the waves are transmitted. The total Doppler effect may, therefore, result from motion of the observer, motion of the source, or motion of the medium. These effects are separately analyzed.

### Commonly Asked Doppler Effect Questions

1. How is the Doppler effect used in everyday life?
2. How does the Doppler effect work?
3. What animals use the Doppler effect?
4. Does Doppler effect apply to light?
5. What is the Doppler effect and how does it affect how you hear a sound?
6. How does the Doppler effect work in space?
7. How do marine animals communicate underwater?
8. Does the Doppler effect apply to sound?
9. How does the Doppler shift affect the colour of a star?
10. How do doctors use the Doppler effect?
11. What are some applications of the Doppler effect?
12. Does the Doppler effect apply to all waves?

Stay tuned with Byju’s to learn more about the Doppler effect derivation, its equations and applications, doppler effect in radar and much more.