Which statement about the Doppler effect is true?

• A. It applies only to sound waves
• B. It applies to all waves and depends on the relative motion between source and observer
• C. It does not depend on motion
• D. It increases wavelength regardless of motion

Answer: (B)

The Doppler effect shows how the observed frequency (and thus wavelength) changes because the source and observer are moving relative to each other. When the source and observer move toward each other, the observer encounters wave crests more frequently, so the observed frequency increases and the wavelength appears shorter. When they move apart, crests arrive less often, so the observed frequency decreases and the wavelength lengthens. This phenomenon is true for all waves, not just sound, though the exact math can differ for light due to relativity. So the best statement captures both the universality of the effect and its dependence on relative motion.