# Classical waves and the Doppler shift

• I
• Phys12
In summary: Not that I remember. So, Doppler shift happens for normal waves (non-quantum) because of change in frequency because of a moving observer or because of change in wavelength and frequency because of the moving source. Is that correct?
Phys12
In the video:

The professor mentions how the wavelength will be the same for a moving observer vs a moving observer for a classical wave like a sound wave. However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a stationary observer?

Phys12 said:
However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a stationary observer?
Think carefully about what the moving observer thinks, he does not necessarily think the wavelength has changed. Or read below:
When a sound source moves relative to the medium, then there is a Doppler-shift of the frequency and a Doppler-shift of the wavelength.

When an observer moves relative to the medium, then there is a Doppler-shift of frequency but no Doppler-shift of wavelength, according to the observer. (Wave-train is moving past me extra fast, that is why extra many wave-crests are passing me each second, says the observer)

(If anyone were to look inside the ear of an observer that is moving towards a sound source, he would find there a wave with a 'shortened' wavelength. But that is not the subject now, right?)

Last edited:
Phys12
jartsa said:
(If anyone were to look inside the ear of an observer that is moving towards a sound source, he would find there a wave with a 'shortened' wavelength. But that is not the subject now, right?)
I don't get this. Why will it be a different wavelength?

Phys12 said:
I don't get this. Why will it be a different wavelength?
Motion of the observer changes the pitch that the ear hears. No wind is blowing inside the ear, so the medium is at rest, in the rest frame of the ear.

Hmm maybe it would be better to consider a car that is driving towards a sound source, and the sound waves in the air inside the car.

Phys12 said:
However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a stationary observer?
Did you look at the Doppler formulas? Does wavelength appear in them?

A.T. said:
Did you look at the Doppler formulas? Does wavelength appear in them?
Not that I remember. So, Doppler shift happens for normal waves (non-quantum) because of change in frequency because of a moving observer or because of change in wavelength and frequency because of the moving source. Is that correct?

## 1. What is a classical wave?

A classical wave is a type of wave that can be described by a mathematical equation known as a wave equation. These waves exhibit characteristics such as amplitude, wavelength, frequency, and velocity, and can travel through various mediums such as air, water, and solids.

## 2. How does the Doppler shift work?

The Doppler shift is the change in frequency or wavelength of a wave due to the relative motion between the source of the wave and the observer. If the source and observer are moving towards each other, the frequency of the wave will increase, resulting in a blueshift. If they are moving away from each other, the frequency will decrease, resulting in a redshift.

## 3. What are examples of classical waves?

Classical waves include sound waves, water waves, electromagnetic waves (such as light and radio waves), and seismic waves. These waves all exhibit similar behaviors and can be described by the same wave equation.

## 4. How is the Doppler shift used in real-life applications?

The Doppler shift has many practical applications, such as in radar technology to measure the speed and direction of moving objects, in medical imaging to detect blood flow and heart rate, and in astronomy to determine the motion of stars and galaxies.

## 5. What is the difference between a transverse wave and a longitudinal wave?

In a transverse wave, the motion of the particles is perpendicular to the direction of wave propagation. Examples of transverse waves include water waves and electromagnetic waves. In a longitudinal wave, the motion of the particles is parallel to the direction of wave propagation. Examples of longitudinal waves include sound waves and seismic waves.

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