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If a police car is travelling away from an observer and its siren is on, the wavelength of the siren will be longer than when the car is moving towards the observer.

Waves - The Doppler Effect and Redshift

In this GCSE Physics quiz you’ll explore red-shift, evidence for an expanding universe, and how light from distant galaxies tells us about their speed and distance.

Explore the Topic →

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Fascinating Fact:

The greater the redshift of a galaxy’s light, the faster it is moving away, suggesting that more distant galaxies recede more quickly.

In GCSE Physics, red-shift is studied as key evidence for an expanding universe. By analysing how the wavelength of light stretches from distant galaxies, students link wave behaviour to cosmology.

Key Terms

Red-shift: The increase in wavelength of light from an object moving away, making the light appear shifted towards the red end of the spectrum.
Doppler effect: The change in observed frequency or wavelength when a source and an observer are moving relative to each other.
Electromagnetic spectrum: The continuous range of all electromagnetic waves, from radio waves through visible light to gamma rays.

Frequently Asked Questions (Click to see answers)

What is red-shift in GCSE Physics?

In GCSE Physics, red-shift is when the wavelengths of light from a distant galaxy appear longer and redder because the galaxy is moving away from us at high speed.

How does red-shift support the Big Bang theory?

Red-shift shows that most galaxies are moving away from us. This suggests space itself is expanding, which supports the Big Bang idea that the universe started from a very hot, dense state.

What does a larger red-shift tell us about a galaxy?

A larger red-shift means the galaxy is moving away faster and is usually further from us. This relationship helps astronomers estimate distances and speeds on a cosmic scale.

Try These Related Quizzes

1 .

It can be observed that there is an increase in the wavelength of light received from distant galaxies. The further away the galaxy is, the faster it is moving away from us and the larger the observed increase in wavelength. What is this effect commonly known as?

Redshift

Blueshift

Whiteshift

Purpleshift

It is known as redshift as the wavelengths of light received from objects travelling away from us are displaced towards the red end of the electromagnetic spectrum. Similarly, light-emitting objects travelling towards us have wavelengths which are shifted towards the blue end of the spectrum and thus this effect is known as blueshift

2 .

If a police car is travelling away from an observer and its siren is on, how will the wavelength be different to when the car is moving towards the observer?

Shorter

Longer

The same

Impossible to tell

Successive waves will arrive later than they would if the car was stationary

3 .

What is the cosmic microwave background radiation (C.M.B.R.)?

A form of sound wave which fills the whole of the Universe and is thought to be a remnant of the early stages of the Universe's life

A form of mechanical wave which fills the whole of the Universe and is thought to be a remnant of the early stages of the Universe's life

A form of electromagnetic radiation which fills the whole of the Universe and is thought to be a remnant of the early stages of the Universe's life

There is no such thing as the C.M.B.R

Scientists think that by studying the C.M.B.R, they can understand some of the conditions which existed in the early parts of our Universe's development

4 .

Where does C.M.B.R come from?

Radiation present shortly after the beginning of the Universe

Radiation generated by humans on Earth

Radiation generated by the sun

Radiation generated by gas giants such as Jupiter and Saturn

It has a temperature close to absolute zero

5 .

How does the frequency and wavelength of a wave source change when it moves towards an observer?

The wavelength increases whilst the frequency decreases

The wavelength decreases whilst the frequency increases

The wavelength increases and the frequency increases

The wavelength decreases and the frequency decreases

If the wavelength of a particular wave decreases, then the frequency will always increase as more waves will pass the observer each second

6 .

Which is the only current theory that explains and accounts for the existence of the C.M.B.R?

Theory of Special Relativity

General Theory of Relativity

Little Bang Theory

Big Bang Theory

This has become the accepted theory of the origin of our Universe but there are still many unanswered questions

7 .

Why does redshift support the idea of the Big Bang theory?

It demonstrates that the Universe is expanding as more distant galaxies are moving away from us

It demonstrates that the Universe is collapsing as more distant galaxies are moving towards us

It does not offer any insight as to whether the theory is true or not

It shows that the Universe in stationary and not expanding or shrinking

The further away the galaxies are, the faster they appear to be moving

8 .

How does the frequency and wavelength of a wave source change when it moves away from an observer?

The wavelength decreases whilst the frequency increases

The wavelength increases whilst the frequency decreases

The wavelength decreases and the frequency decreases

The wavelength increases and the frequency increases

For light, the effect is only noticeable when the movement is at extremely high speeds

9 .

What is the Doppler effect?

The Doppler effect is the change in the observed wavelength and frequency of a source which is stationary relative to an observer

The Doppler effect is the change in the observed wavelength and frequency of a source which is moving relative to an observer

The Doppler effect is the change only in the observed wavelength of a source which is stationary relative to an observer

The Doppler effect is the change only in the observed frequency of a source which is stationary relative to an observer

Both the frequency and the wavelength change when a source is moving relative to an observer. You can hear this effect yourself by listening carefully to traffic going past on a main road. You can hear how the sound changes when the vehicles are travelling towards you and away from you

10 .

If a police car is travelling towards an observer and its siren is on, will the frequency of the siren be higher, lower or the same as when the car is moving away from the observer?

Lower

Higher

The same

Impossible to tell

Successive waves will arrive sooner than if the car was stationary

You can find more about this topic by visiting BBC Bitesize - The Expanding Universe

Author: Martin Moore