What is the Doppler effect?

• A. Reflection
• B. Doppler effect
• C. Perception in music
• D. Closure

Answer: (B)

The Doppler effect is the change in observed frequency (and thus pitch for sound) that occurs when the source of waves and the observer move relative to each other. If the source approaches, waves are compressed and you hear a higher frequency; if it moves away, waves are stretched and you hear a lower frequency. The actual speed of the wave in its medium stays the same—the change is in the spacing of the wavefronts as the source and observer move. A common example is an ambulance siren: as it comes toward you the pitch seems higher, and after it passes and moves away it sounds lower. This effect also appears with light, causing blueshift when moving toward you and redshift when moving away, though that uses relativistic formulas for light. It’s not about reflection, which is bouncing waves off surfaces, nor about general perception in music or perceptual closure, which are different ideas.