Highworth Physics - The Doppler Effect

Front 1

What is the Doppler effect?

Back 1

When a sound source (like an ambulance or police car) goes past, its sound is high-pitched as it comes towards you, then becomes low-pitched as it moves away.

This effect, where there is a change in frequency and wavelength, is called the Doppler effect.

Front 2

What happens to the wavelength and frequency as a source moves towards an observer?

Back 2

The observed wavelength decreases and the frequency increases.

Front 3

What happens to the wavelength and frequency as a source moves away from an observer?

Back 3

The observed wavelength increases and the frequency decreases.

Front 4

What type of waves does the doppler effect occur with?

Back 4

This happens to all waves, including electromagnetic waves.

Front 5

What is red shift?

Back 5

When an object moves away from an observer, its light is affected by the Doppler effect. The wavelength increases.

Front 6

What is blue shift?

Back 6

When an object comes towards an observer, its light is affected by the Doppler effect. The wavelength decreases.

Front 7

How do black lines in the spectrum of the light from the Sun help us to determine if distant stars are moving towards or away from us?

Back 7

The black lines in the spectrum of the light from the Sun is where helium has absorbed light. These lines form the absorption spectrum for helium.

When we look at the spectrum of a distant star, the absorption spectrum is there, but the pattern of lines has moved (or ‘shifted’) towards the red end of the spectrum.

Front 8

Astronomers have found that the further from us a star is, the more its light is red-shifted.

What does this tells us about distant galaxies?

Back 8

This tells us that distant galaxies are moving away from us, and that the further away a galaxy is, the faster it is moving away.

Front 9

Describe the changes to the pitch of the sound heard by people on the platform as a train passes.

Back 9

The pitch will be increased as the train approaches and decreased as it moves away.

Front 10

A light source is moving so that an observer sees the light ‘red-shifted’.

(a)What happens to the wavelength and the frequency of the observed light?

(b) Draw a diagram to show how the light source is moving relative to the observer.

Back 10

(a) The wavelength is longer and the frequency lower.

(b) Diagram should have an arrow showing the source moving away from the observer; the source and observer must be labelled.

Front 11

Two go-karts are being driven along a race track at the same speed. Explain whether the drivers notice any change in the sound of:

(a) their own go-kart

(b) the other go-kart.

Back 11

(a) The drivers would notice no changes because they are not moving relative to the go-kart.

(b) The drivers would notice no changes because they are travelling at the same speed, so not moving relative to each other.

Front 12

A satellite orbits the Earth transmitting a beam of microwaves.

(a) Describe how the frequency of the microwaves received by a detector changes as the satellite passes overhead.

(b)What is this effect called?

Back 12

(a) The frequency increases as the satellite approaches, and decreases as it moves away.

(b) This is called the Doppler effect.

Front 13

Explain how a driver of a moving car hears a siren but can NOT hear the pitch of the siren changing.

Back 13

There is no relative motion between the car and the siren (the car is travelling at the same speed and in the same direction as the siren).

Front 14

A laser beam is used to measure the speed of a car. The light is directed at the car and reflected back to a detector.

(a) How does the wavelength change if the car is: (i) approaching the laser? (ii) travelling away from the laser?

(b) The car is travelling towards the laser and it speeds up. What difference will this make to the wavelength of the light detected?

(c) The laser and detector are now driven at the same speed behind the first car. Explain whether there are any changes to the wavelength of the light received.

Back 14

(a) (i) Wavelength decreases. (ii) Wavelength increases.

(b) Shift in wavelength will be greater.

(c) There will be no change in the wavelength because the observer is not moving relative to the source of the sound; they are staying the same distance apart because they are travelling at the same speed.

Front 15

Explain why the motion of a siren affects the pitch of the sound heard by a stationary observer.

How can this effect be demonstrated in the science laboratory?

Back 15

When the source is moving towards the observer the waves moving towards the observer are squashed together. So, the wavelength decreases and frequency increases. As the speed of the waves has not changed, because the wavelength decreases the frequency increases, the pitch of the sound has increased. When the source is moving away from the observer the waves are stretched out, so he observes a longer wavelength and lower frequency/pitch.

An experiment that could be done in the laboratory – e.g. whirling a small radio or other named sound source around on a piece of string.

Front 16

In Doppler ultrasound scans of the heart, computers use the Doppler effect to produce an image of the blood flow inside the heart. Ultrasound waves are sent towards the heart and are reflected back. Explain how the Doppler effect can be used to give information about the speed and direction of the moving blood flow in the heart.

Back 16

If blood is moving towards the ultrasound source, reflected ultrasound waves will have decreased wavelength and increased frequency.

In contrast, if blood is moving away from ultrasound source reflected ultrasound waves will have increased wavelength and decreased frequency.

If blood is moving faster there will be a larger shift in frequency and wavelength.

Front 17

What are the key points to remember?

Back 17

If a source of waves moves towards the observer the wavelength decreases and the frequency increases.

If a source of waves moves away from the observer the wavelength increases and the frequency decreases.

The Doppler effect happens for all waves including sound, light and microwaves.

For electromagnetic waves a decrease in wavelength is called blue shift, and an increase in wavelength is called red shift.