If the source of a wave moves toward an observer, what happens to the observed frequency?

• A. The observed frequency increases when the source moves toward the observer
• B. The observed frequency decreases when the source moves toward the observer
• C. The observed frequency remains the same regardless of motion
• D. The observed frequency is unrelated to motion

Answer: (A)

When the source of a wave moves toward an observer, the waves are emitted with crests that are closer together along the line to the observer. Since the wave speed in the medium stays the same, this shorter spacing means a smaller wavelength reaching the observer. Frequency is linked to speed and wavelength by f = v/λ, so a smaller λ produces a higher observed frequency. In other words, approaching motion compresses the wavefronts and raises the pitch you hear. If the source were moving away, the crests would be spaced farther apart and the observed frequency would decrease.