NEET Physics — Doppler Effect

Chapter Overview & Weightage

Doppler effect for sound waves is a small sub-topic within “Waves” but consistently delivers 1 question per NEET paper — straightforward marks if you nail the sign convention.

Year	NEET Qs
2024	1
2023	1
2022	1
2021	1

The trickiness is entirely in the sign convention. Get that right, and the answer drops out in 60 seconds.

Key Concepts You Must Know

Apparent frequency: the frequency heard by the observer when source and/or observer is moving relative to the medium.
Speed of sound ( $v$ ): about 340 m/s in air at room temperature; this is the propagation speed in the medium, not the source’s speed.
Source velocity ( $v_s$ ): velocity of the source of sound.
Observer velocity ( $v_o$ ): velocity of the observer.
Sign convention: take the direction from observer to source as positive.

Important Formulas

$f' = f \cdot \frac{v + v_o}{v + v_s}$

(This uses the convention: $v_o$ positive if observer moves toward source; $v_s$ positive if source moves away from observer.)

Equivalent form (clearer for many students):

$f' = f \cdot \frac{v \pm v_o}{v \mp v_s}$

Numerator: + when observer moves toward source, − when away.
Denominator: − when source moves toward observer, + when away.

$f' = f \cdot \frac{v}{v - v_s}$ (source approaching)

$f' = f \cdot \frac{v}{v + v_s}$ (source receding)

$f' = f \cdot \frac{v + v_o}{v}$ (observer approaching)

$f' = f \cdot \frac{v - v_o}{v}$ (observer receding)

Solved Previous Year Questions

PYQ 1 (NEET 2024)

A train approaches a stationary observer at 30 m/s while sounding a horn of frequency 500 Hz. The speed of sound is 340 m/s. Apparent frequency heard?

Solution: Source approaching observer:

$f' = 500 \cdot \frac{340}{340 - 30} = 500 \cdot \frac{340}{310} \approx 548.4\text{ Hz}$

PYQ 2 (NEET 2023)

A car moving at 20 m/s passes a stationary siren emitting 1000 Hz. As the car moves away, the apparent frequency heard by a passenger? ( $v = 340$ m/s.)

Solution: Observer (car passenger) moving away from source:

$f' = 1000 \cdot \frac{340 - 20}{340} = 1000 \cdot \frac{320}{340} \approx 941\text{ Hz}$

PYQ 3 (NEET 2022)

Source moves at 10 m/s toward an observer who moves at 5 m/s away from the source. Source frequency 200 Hz, $v = 340$ m/s. Apparent frequency?

Solution: Source approaching (− in denominator), observer receding from source (− in numerator):

$f' = 200 \cdot \frac{340 - 5}{340 - 10} = 200 \cdot \frac{335}{330} \approx 203\text{ Hz}$

Difficulty Distribution

Easy (60%): Straightforward source moving or observer moving (one of them stationary).
Medium (30%): Both moving simultaneously, often in same or opposite directions.
Hard (10%): Reflection from a moving wall (echo problems), or wind affecting effective sound speed.

Expert Strategy

Sign-by-direction method: Don’t memorise ”+/−” rules abstractly. Instead, ask: does this motion bring source and observer closer? If yes, frequency increases (use signs that boost numerator and reduce denominator). If they’re moving apart, frequency decreases.

Reflection problems: When sound reflects off a moving wall, treat the wall as first an observer (hears apparent frequency $f_1$ ) and then a source (re-emits at $f_1$ , the original observer hears $f_2$ ). Two-step Doppler. NEET 2017 had this pattern.

Common Traps

Trap 1: Wind correction. If wind blows from source to observer with speed $w$ , replace $v$ in the formulas with $v + w$ . If wind blows opposite, use $v - w$ . NEET sometimes hides this in the problem.

Trap 2: Doppler doesn’t apply for transverse motion. If a source moves perpendicular to the line of sight (purely tangential motion at the moment in question), there is no Doppler shift in classical sound. Apparent frequency = actual frequency.

Trap 3: Symmetric vs asymmetric Doppler. The sound Doppler is not symmetric: source moving toward observer at $v_s$ gives a different shift than observer moving toward source at $v_s$ . (Light Doppler is symmetric due to relativity, but that’s outside NEET.)

For NEET, $v = 340$ m/s is the standard value. Some questions use $v = 330$ or $350$ — read carefully. Also: NEET problems are usually one-step (only source or only observer moves). Save the two-moving-objects formula for the rare hard question.