Doppler Effect Applies to All Waves

Steps:

• Recall the Doppler effect: change in wave frequency or wavelength due to relative motion between source and observer.
• Identify wave types: includes mechanical waves (sound) and electromagnetic waves (light), plus others like water waves.
• Verify examples: observed in sound (e.g., siren pitch), light (redshift in astronomy), and water waves (boat wakes).
• Conclude universality: no restriction to specific wave categories in the definition.

Why A is correct:

• The Doppler effect is a general property of waves, described by the formula $f' = f \frac{v \pm v_o}{v \mp v_s}$, applicable to any propagating wave where source or observer moves.

Why the others are wrong:

• B: Excludes sound waves, but Doppler shift is observed in audible frequencies like ambulance sirens.
• C: Limits to sound and light, ignoring other waves like seismic or water waves where it's also observed.
• D: Restricts to sound, but it's evident in light (e.g., stellar spectra) and other wave types.

Final answer: A