2.5. Doppler Effect (C5 - AHL)

• The Doppler effect describes the apparent change in frequency of a wave when there is relative motion between the source and the observer.

When:

• • The source approaches, the waves are compressed → higher frequency (pitch rises).

  • The source moves away, the waves are stretched out → lower frequency (pitch drops).

• It’s not the actual frequency changing — it’s how the observer perceives it, due to the motion affecting the wavefront spacing.

• Radar and speed guns: Measure how fast cars or baseballs are moving.

• Medical imaging (Doppler ultrasound): Measure blood flow velocity in arteries.

• Astronomy: Determine if stars or galaxies are moving toward or away from Earth (redshift/blueshift).

• Weather radar: Track movement of storm systems.

• Navigation and sonar: Submarines and ships use Doppler shift to detect moving objects under water.

Mechanical waves (like sound):

• Require a medium (air, water, etc.).

• Doppler shift depends on motion relative to the medium, and you must consider both source and observer velocities relative to the medium.

• Speed of sound depends on medium properties.

Electromagnetic waves (like light):

• Can travel through a vacuum.

• Doppler shift is due to relative motion only, and at high speeds, relativistic effects become important.

• The speed of light is constant in all inertial frames (according to special relativity), so the equations differ from sound.