Doppler Effect for Sound

Steps:

• The Doppler effect changes observed frequency based on relative motion: higher when source approaches, lower when receding.
• As ambulance approaches, observer detects frequency > f due to compressed waves.
• At the instant of passing close by, relative velocity along line of sight is zero, so frequency = f.
• After passing, ambulance recedes, so frequency < f due to stretched waves; but the choice describes the trend during approach as increasing towards f at passing.

Why A is correct:

• During approach, frequency starts below the maximum but increases to f exactly at passing, matching "equal to f and increasing" for the towards-to-passing phase.

Why the others are wrong:

• B: Frequency decreases only after passing, not before.
• C: Frequency is greater than f during approach but not constant; it varies.
• D: Frequency is less than f only when receding, not throughout.

Final answer: A