Key Concepts: Doppler Effect

The Doppler Effect

• The change in observed frequency (and wavelength) due to relative motion between a source and observer.

For Sound

• Source approaching: Wavelength compressed → higher frequency (higher pitch).
• Source receding: Wavelength stretched → lower frequency (lower pitch).
• Formula: f' = f × (v ± v_o)/(v ∓ v_s), where v is speed of sound, v_o is observer speed, v_s is source speed.

For Light

• Blue shift: Light from approaching objects has shorter wavelength (shifted blue).
• Red shift: Light from receding objects has longer wavelength (shifted red).
• Astronomical red shift provides evidence that the universe is expanding.

Applications: Radar/speed guns, medical ultrasound, measuring stellar velocities.