Doppler Effect

Physics 11

Doppler Effect

When an source is moving with respect to an observer (or vice versa) the frequency of the sound will shift due to the Doppler Effect

As a result, it is possible to determine whether an object is moving toward or away from us if we know the reference pitch

Doppler Effect

Doppler Effect

http://webphysics.davidson.edu/applets/applets.html

The observed frequency (f) is related to the emitted frequency (f0) where vs is the speed of sound

vr is the speed of receiver (positive if moving toward emitter)

ve is the speed of emitter (positive if moving away from receiver)

es

rs

vv

vvff 0

Doppler Effect

The siren of a police car at rest emits at a predominant frequency of 1600Hz. What frequency will you hear if you at rest and the police car moves at 25.0m/s: Toward you? Away from you?

What happens to the previous result if the siren is at rest and you move at 25.0m/s toward and away from the siren?

Doppler Effect

1726Hz1491Hz1717Hz1483Hz

Doppler Effect

What happens if:You move toward the siren at 25.0m/s while

the siren moves away at 25.0m/sYou move toward the siren at 25.0m/s while

the siren moves toward you at 25.0m/sYou move away from the siren at 25.0m/s

while the siren moves toward you at 25.0m/sYou move away from the siren at 25.0m/s

while the siren moves away from you at 25.0m/s

Doppler Effect

What happens if:No shift1851HzNo shift1383Hz

With Light?

We can see the same behaviour with light; since light (in a vacuum) must always travel at the speed of light (3.0x108m/s)

However, since nothing can travel faster than the speed of light (in a vacuum) it is impossible to see behaviour akin to a sonic boom with light (in a vacuum)

Red and Blue Shift

When the wavelength or frequency of light is changed, so is its “colour”

For visible light, this means that when an emitter is moving away from Earth, the wavelength observed is increased and light is said to be red shifted

When light is emitter by a body moving toward Earth, wavelength is decreased and we say that it is blue shifted

Doppler Effect

The observed frequency (f) is related to the emitted frequency (f0) where c is the speed of light v is the speed of the

emitter relative to the observer (positive if source is moving toward)

vc

vcff

0

Example

A 650nm photon from the Sun is observed on Earth to have a wavelength of 655nm. How fast is the Sun moving relative to the Earth and is it moving toward or away?

Example

-2.60x106m/sAway

Practice A police siren emits a wave with a frequency of

300Hz. If the speed of sound in air is 340m/s, determine:

The wavelength of waves if the siren is at rest in the air. (1.13m)

If the siren is moving at 30m/s, find the wavelengths of the waves if it is moving toward or away from an observer. (1.03m, 1.23m)

If the siren is moving away from an observer, what frequency do they hear? (276Hz)

If the siren is at rest and the observer is away from the source at 30m/s, what frequency do they hear? (274Hz)

If the siren is moving away from the observer at a speed of 45m/s and the listener is moving toward the siren with a speed of 15m/s, what frequency does the listener hear? (277Hz)

Practice

The police car is moving toward a warehouse door at 30m/s and the sound is reflected from the door back to the police car. What does an observer in the car hear? (358Hz)

How fast must a galaxy be travelling if a 525nm photon is measured to have a wavelength of 495nm? Is it moving toward Earth or away?