Doppler Effect: Lowest Frequency When Source Moves Away

Steps:

• Recall Doppler formula for sound: observed frequency $f' = f \frac{v}{v \pm v_s}$, where $v$ is sound speed, $v_s$ is source speed (+ if source toward observer, - if away).
• Lowest $f'$ occurs when denominator is largest, i.e., source moving directly away ($v - v_s$).
• Analyze diagrams: compare source velocity direction relative to stationary observer.
• Identify case where source recedes directly from observer for minimal $f'$.

Why D is correct:

• In D, source moves directly away, maximizing denominator in Doppler formula, yielding lowest $f' < f$.

Why the others are wrong:

• A: Source approaches observer, minimizing denominator, so $f' > f$.
• B: Source moves perpendicular to line of sight, no radial velocity component, so $f' = f$.
• C: Source approaches at an angle, partial toward component, so .

Final answer: D