Doppler Effect for Moving Source

Steps:

• Source approaching observer: relative speed reduces effective wavelength, so f' = f * v / (v - v_s) > f, constant for uniform speed.
• At passing: source velocity component reverses instantly from towards to away.
• Source receding: f' = f * v / (v + v_s) < f, constant low value.
• Graph shows step change in f' at t when source passes.

Why B is correct:

• B depicts constant high f' before passing, abrupt drop to constant low f' after, per Doppler formula's dependence on velocity direction.

Why the others are wrong:

• A: Shows gradual linear decrease, but direct path yields constant f' segments, not continuous ramp.
• C: Symmetric curve implies smooth transition, but shift is discontinuous at passing.
• D: No abrupt change or incorrect shape, failing to capture velocity reversal.

Final answer: B