Define the Doppler effect.

The Doppler effect is the change in observed frequency or wavelength of a wave when there is relative motion between the source and the observer.

State the Doppler equation for electromagnetic radiation and define each term.

Δf/f = Δλ/λ ≈ v/c, where Δf is the change in frequency, f is the original frequency, Δλ is the change in wavelength, λ is the original wavelength, v is the relative velocity of the source, and c is the speed of light.

How can the recession velocity of a galaxy be determined from observations?

By measuring the redshift of the galaxy's spectral lines and applying the Doppler equation to calculate the recession velocity.

State Hubble's law and define each term in the equation.

v ≈ H₀d, where v is the recession velocity of a galaxy, H₀ is the Hubble constant, and d is the distance to the galaxy.

OCR A-Level Physics A H556

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Doppler shift and Hubble's law

3 things to know
Papers 1, 2 and 3

Overview

Recall questions on the Doppler effect for electromagnetic radiation, recession velocity from redshift, Hubble's law and the Hubble constant, with calculations.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What happens to the observed frequency of electromagnetic radiation when the source moves towards the observer?

Key term

What happens to the observed wavelength of electromagnetic radiation when the source moves away from the observer?

Numbers

Calculate the recession velocity when change in wavelength = 19 nm, rest wavelength = 460 nm, speed of light = 3.00 × 10⁸ m s⁻¹.

Questions from this subtopic

Each one with the model answer underneath.

1
Define the Doppler effect.
Model answer
The Doppler effect is the change in observed frequency or wavelength of a wave when there is relative motion between the source and the observer.
2
What happens to the observed frequency of electromagnetic radiation when the source moves towards the observer?
Model answer
The observed frequency increases, causing a blueshift.
3
What happens to the observed wavelength of electromagnetic radiation when the source moves away from the observer?
Model answer
The observed wavelength increases, causing a redshift.
4
State the Doppler equation for electromagnetic radiation and define each term.
Model answer
Δf/f = Δλ/λ ≈ v/c, where Δf is the change in frequency, f is the original frequency, Δλ is the change in wavelength, λ is the original wavelength, v is the relative velocity of the source, and c is the speed of light.
5
How can the recession velocity of a galaxy be determined from observations?
Model answer
By measuring the redshift of the galaxy's spectral lines and applying the Doppler equation to calculate the recession velocity.
6
State Hubble's law and define each term in the equation.
Model answer
v ≈ H₀d, where v is the recession velocity of a galaxy, H₀ is the Hubble constant, and d is the distance to the galaxy.

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