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Astronomy

The Doppler effect and red-shift in galaxies

  • 3 things to know
  • Paper 1

Overview

How observed wavelength and frequency change for a source moving away or towards an observer, the ambulance siren, what red-shift is, and how it grows with a galaxy's distance and speed.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Evaluation

Define red-shift, describe how a galaxy's red-shift relates to its distance and to how fast it moves away, and compare the red-shifts of galaxy A and galaxy B if A is about twice as far from the Earth.

Key term

Explain why the pitch of a siren on an ambulance seems to drop as the ambulance drives past a listener.

Key term

What happens to the observed wavelength of waves from a source that is moving away from an observer?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What happens to the observed wavelength of waves from a source that is moving away from an observer?

    Model answer

    The observed wavelength increases.

  2. 2

    What happens to the observed frequency of waves from a source that is moving away from an observer?

    Model answer

    The observed frequency decreases.

  3. 3

    What happens to the observed wavelength and frequency of waves from a source that is moving towards an observer?

    Model answer

    The observed wavelength decreases and the observed frequency increases.

  4. 4

    Explain why the pitch of a siren on an ambulance seems to drop as the ambulance drives past a listener.

    Model answer

    While the ambulance approaches, the sound waves reach the listener at a higher frequency, and once it is moving away the frequency received is lower, so the pitch falls.

  5. 5

    When a wave source moves relative to an observer, does the frequency emitted by the source itself change?

    Model answer

    No, the source emits the same frequency as before and only the frequency received by the observer is different.

  6. 6

    State the condition needed for an observer to notice a change in the frequency of waves coming from a source.

    Model answer

    The source and the observer must be moving relative to one another.

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