Question 1a

Marks: 3

(a)

Define the Doppler effect.

[3]

Question 1b

Marks: 4

The diagram shows two people observing the same light source, which is in motion. The light is moving to the right.

9-5-ib-hl-sqs-easy-q1a-question

(b)

Complete the boxes on the diagram with words or phrases from the list below.

[4]

higher frequency	lower frequency	phase difference	wavelength
blue shift	relative motion	star	red shift

Question 1c

Marks: 7

The following text is about the Doppler effect.

(c)

Complete the following sentences by circling the correct words:

When a source starts to move away from / towards the observer, the wavelength of the wave broadens.

For sound waves, sound therefore appears at a higher / lower frequency to the observer.

For light waves, the light shifts towards the blue / red end of the electromagnetic spectrum due to its lower / higher frequency.

When a source starts to move away from / towards the observer, the wavelength of the wave shortens.

For sound waves, sound therefore appears at a higher / lower frequency to the observer.

For light waves, the light shifts towards the blue / red end of the electromagnetic spectrum due to its lower / higher frequency.

This is because blue / red light has a longer wavelength than blue / red light.

[7]

Question 1d

Marks: 2

(d)

State two uses of the Doppler effect.

[2]

Question 2a

Marks: 2

(a)

Define redshift.

[2]

Question 2b

Marks: 5

For non-relativistic galaxies, Doppler redshift can be calculated using:

$\frac{Δ λ}{λ} = \frac{Δ f}{f} = \frac{v}{c}$

(b)

(i)

State what is meant by a non-relativistic galaxy.

[1]

(ii)

Define the symbols

λ

,

f

,

v

and

c

in the equation above.

[4]

Question 2c

Marks: 3

The shift in wavelength can be identified using spectral lines in an absorption spectrum.

The following absorption spectrum is generated using light from a source in a laboratory:

iiyb8v8q_9-5-ib-hl-sqs-easy-q2c-question

(c)

Sketch the spectrum obtained using light from a galaxy moving away from the Earth.

[2]

Question 2d

Marks: 5

An absorption line in the spectrum of light from a source in a lab in part (c) has a frequency of 5.783 × 10¹⁴ Hz. The same line in the spectrum of light from a distant galaxy has a frequency of 5.791 × 10¹⁴ Hz.

(d)

Calculate the speed of the distant galaxy in relation to the Earth.

[5]

Question 3a

Marks: 4

This question is about redshift.

(a)

Complete the following sentences by filling in the correct words in the gaps:

After the discovery of Doppler redshift, astronomers began to realise that almost all the galaxies in the universe are ________.

This led to the idea that the universe is must be ________.

This caused the light waves to ________ ________ as they travel through space, shifting them towards the ________ end of the spectrum.

The more shifted the light from a galaxy is, the ________ the galaxy is moving away from Earth.

[4]

Question 3b

Marks: 4

The following equation is used for calculations where the observer is stationary and the source is moving:

$f' = f (\frac{v}{v \pm u_{s}})$

(b)

Define each of the terms,

f

,

f'

,

v

and

u_{s}

in the equation above and give the correct unit.

[4]

Question 3c

Marks: 2

(c)

State the equation used if the source is stationary and the observer is moving, defining any terms which differ from the equation in part (b).

[2]

Question 3d

Marks: 3

A distant galaxy is receding from the Earth at a velocity of 4.5 × 10⁵ m s⁻¹. The galaxy emits light of frequency 5.5 × 10¹⁴ Hz.

(d)

Using the equation

f' = f (\frac{v}{v + u_{s}})

, calculate the frequency of this light as observed from the Earth.

[3]

Question 4a

Marks: 3

The equation for the Doppler effect can be written as follows:

$λ' = λ (1 \pm \frac{u_{s}}{v})$

(a)

Draw lines to match the symbol with the correct definition.

[3]

Question 4b

Marks: 2

(b)

State the conditions for the equation in part (a) which result in the term in brackets being:

(i)

1 - \frac{u_{s}}{v}

[1]

(ii)

1 + \frac{u_{s}}{v}

[1]

Question 4c

Marks: 3

An ambulance is racing towards the scene of an emergency at a speed of 20 m s^−1. The siren on the ambulance produces a sound of wavelength 0.2 m. The speed of sound in air is 340 m s⁻¹.

(c)

Using the equation in part (a), determine the wavelength of sound as heard by an observer at the scene.

[3]

Question 4d

Marks: 2

(d)

State and explain the effect of the wavelength of the sound calculated in part (c) with reference to what the observer hears.

[2]

Question 5a

Marks: 2

A team of naturalists are researching the movement of whales in the ocean. They plan to calculate the velocity of a whale using the Doppler effect. The whale pod is moving towards the research team.

(a)

State an equation which will allow the researchers to investigate the velocity of the whales using the frequency of sound in water using the following variables:

λ, λ', u_s, v, f, f'

You may use each variable once, more than once or not at all.

[2]

Question 5b

Marks: 2

(b)

State and explain the difference between the sound produced by the whales and sound observed by the researchers.

[2]

Question 5c

Marks: 2

The whales are producing a monotone sound of frequency 50 Hz. The speed of sound in water is 1480 m s⁻¹. The whales are moving at 10 m s⁻¹.

(c)

Use the equation from part (a) to calculate the frequency of the sound from the whales as observed by the research team.

[2]

Question 5d

Marks: 3

The whales stop moving when they detect the boat. The researchers, not wanting to scare the whales, start to move away from the pod, accelerating over a period of time until reaching a steady speed of 20 m s⁻¹. The boat comes to a stop 500 m from the whale pod.

(d)

Order the phrases below from 1-3 to describe how the observed sound from the whales changes over this period.

The frequency of sound observed decreases gradually
The sound observed is 50 Hz
The sound observed is at a steady frequency lower than 50 Hz

[3]

DP IB Physics: HL

Topic Questions

9.5 Doppler Effect

Question 1a

Question 1b

Question 1c

Question 1d

Question 2a

Question 2b

Question 2c

Question 2d

Question 3a

Question 3b

Question 3c

Question 3d

Question 4a

Question 4b

Question 4c

Question 4d

Question 5a

Question 5b

Question 5c

Question 5d

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