The radar used by police to check over-speeding vehicles works on the principle of

Correct — C, Doppler effect. A speed radar sends out a radio wave, the moving vehicle reflects it back, and the returned wave comes home at a slightly different frequency. That shift in frequency caused by relative motion between source and observer is the Doppler effect, and its size is proportional to the vehicle's speed along the line of sight, so measuring the shift measures the speed. The same principle reads the speed of a cricket ball, of blood in an artery and of a distant galaxy.

• (a)Raman effect — The Raman effect is the change in wavelength when light is scattered inelastically by molecules. It identifies chemical bonds, and the shift depends on the molecule rather than on how fast anything is moving.
• (b)Induction effect — Electromagnetic induction is what runs transformers and generators. A traffic radar has no coil moving through a field.
• (d)Coulomb effect — There is no 'Coulomb effect'; Coulomb's law gives the force between two static charges, which has nothing to do with measuring a vehicle's speed.

When a wave source and an observer move relative to each other, the observed frequency differs from the emitted one. Approaching motion compresses the arriving wavefronts and raises the frequency; receding motion stretches them and lowers it. A speed gun exploits this twice over, because the vehicle first receives a shifted wave and then re-radiates it, and the returning signal carries a doubled shift.

The everyday version is the drop in pitch as an ambulance passes. A radar gun works on the same physics with radio waves instead of sound, which is why the reading depends on the angle — a gun aimed obliquely at the road measures only the component of velocity along its line of sight and under-reads. Astronomers use the shift the other way round: the light of distant galaxies arrives stretched towards the red, and that redshift is the observation on which the expanding universe rests.

• The Doppler effect is the change in observed frequency caused by relative motion between a wave source and an observer.
• Approach raises the observed frequency; recession lowers it.
• A speed radar measures the frequency shift of its own reflected signal, and that shift is proportional to the target's speed along the beam.
• Doppler ultrasound uses the same principle to measure blood flow, and Doppler weather radar to track rainfall and wind inside a storm.
• The redshift of light from distant galaxies is the astronomical form of the effect and is central evidence for an expanding universe.

• Mixing up the Doppler effect with the Raman effect — one is about motion, the other about molecular structure.
• Assuming a speed gun works by timing the echo; that is how a rangefinder measures distance, not speed.
• Forgetting that an obliquely aimed radar under-reads, because only the component of velocity along the beam counts.

Named-effect matching. The examiner offers three other physicists' effects and expects the candidate to attach each name to its phenomenon.

• practice — not a real PYQ

  The apparent change in the pitch of an ambulance siren as it passes a stationary listener is explained by

  • (a)reflection of sound
  • (b)the Doppler effect
  • (c)resonance
  • (d)diffraction of sound

  Answer(b) the Doppler effect — the pitch is higher while the ambulance approaches and lower once it has gone past.

• practice — not a real PYQ

  The redshift observed in the light from distant galaxies indicates that they are

  • (a)approaching the Earth
  • (b)receding from the Earth
  • (c)stationary relative to the Earth
  • (d)rotating rapidly

  Answer(b) receding from the Earth — recession stretches the arriving light towards longer, redder wavelengths.