Doppler Effect

Definition

The apparent change in the frequency (or wavelength) of a wave when the source and the observer move relative to each other.

Key points

• As a source approaches, waves bunch up and the observed frequency rises (higher pitch); as it recedes, waves stretch out and frequency falls (lower pitch).
• A common example is the changing pitch of an ambulance or train siren as it passes by.
• It applies to all waves, sound and light; for light from distant galaxies the shift toward longer wavelengths is the "redshift" that supports the expanding-universe idea.
• Practical uses include Doppler radar for weather and speed guns for traffic enforcement, and Doppler ultrasound in medicine to measure blood flow.
• The effect is named after the Austrian physicist Christian Doppler, who proposed it in 1842.

Why it matters for CAPF

The Doppler effect (siren pitch change, radar speed guns, redshift) is a standard wave-physics fact, and Doppler radar and speed detection connect to law enforcement and surveillance.

Common confusion

The actual frequency of the source does not change; only the frequency received by the observer changes because of relative motion. The pitch is higher on approach and lower on recession, not the reverse.

Related concepts

concept electromagnetic spectrum, concept ultrasound and sonar

Parent note

physics everyday