Doppler Effect & Redshift

Overview

This lesson explains why a moving source is heard at a different frequency from the frequency it emits. The assessed AS model is for a moving sound source and a stationary observer, with redshift used as a useful extension of the same frequency-shift idea.

What You Need to Know

• When a source moves towards an observer, successive wavefronts are closer together, so the observed wavelength is smaller and the observed frequency is higher.
• When a source moves away, successive wavefronts are farther apart, so the observed wavelength is larger and the observed frequency is lower.
• For sound from a moving source, use f_o = f_s v / (v +/- v_s), where v is the wave speed and v_s is the source speed.
• The sign is chosen from the physical situation: approaching gives a higher observed frequency; receding gives a lower observed frequency.
• Redshift describes light received with a longer wavelength than emitted. Treat it here as a qualitative connection unless a question gives the extension explicitly.

How to Work Through It

1. Use wavefront diagrams to compare a stationary source with an approaching and receding source.
2. Translate the diagram into frequency and wavelength changes heard by a stationary observer.
3. Practise choosing the correct sign in the Doppler equation before substituting values.
4. Link lower observed frequency and longer wavelength to the idea of redshift.

Check Your Understanding

• Why does an approaching source give a higher observed frequency?
• How can you tell from the answer whether the sign choice was sensible?
• Which speed in the equation is the wave speed, and which is the source speed?
• What is the wavelength change when light is described as redshifted?

Common Mistakes

• Applying this AS equation to a moving observer instead of a moving source.
• Choosing the sign mechanically instead of checking whether the answer should be higher or lower.
• Confusing source frequency with observed frequency.
• Treating redshift as a change in the speed of light rather than a change in wavelength and frequency.

Next Steps

• Complete the Doppler practice questions with a short written reason for each sign choice.
• Review wavefront spacing because the same reasoning helps with later wave diagrams.