Doppler Effect
Apparent frequency for moving source and observer in sound. (Physics › Oscillations and Waves, NEET UG syllabus.)
What is Doppler Effect?
The apparent change in the frequency and wavelength of a wave perceived by an observer moving relative to the source of the wave.
Key formula / rule: General Doppler Effect Formula (Sound)
Key points
- Define the Doppler Effect and explain its physical basis.
- State the general formula for the apparent frequency due to the Doppler Effect for sound waves.
- Apply correct sign conventions for source and observer velocities in different scenarios.
- Calculate the apparent frequency when the source, observer, or both are in motion.
Common exam trap
Incorrectly applying sign conventions for source and observer velocities in the formula.
Definitions
- Term
Doppler Effect
- Meaning
The apparent change in the frequency and wavelength of a wave perceived by an observer moving relative to the source of the wave.
- Term
Apparent Frequency (f')
- Meaning
The frequency of a wave as perceived by an observer, which may differ from the actual frequency of the source due to relative motion.
Learning objectives
Define the Doppler Effect and explain its physical basis.
State the general formula for the apparent frequency due to the Doppler Effect for sound waves.
Apply correct sign conventions for source and observer velocities in different scenarios.
Calculate the apparent frequency when the source, observer, or both are in motion.
Identify real-world applications of the Doppler Effect.
Formulae
- Name
General Doppler Effect Formula (Sound)
- Note
Where 'v' is speed of sound, 'vo' is observer speed, 'vs' is source speed. For 'vo': use '+' if observer moves towards source, '-' if away. For 'vs': use '-' if source moves towards observer, '+' if away.
- Expression
f' = f * (v ± vo) / (v ∓ vs)
- Name
Source moving towards stationary observer
- Note
Apparent frequency increases.
- Expression
f' = f * v / (v - vs)
- Name
Source moving away from stationary observer
- Note
Apparent frequency decreases.
- Expression
f' = f * v / (v + vs)
- Name
Observer moving towards stationary source
- Note
Apparent frequency increases.
- Expression
f' = f * (v + vo) / v
- Name
Observer moving away from stationary source
- Note
Apparent frequency decreases.
- Expression
f' = f * (v - vo) / v
Prerequisites
Basic understanding of wave properties (frequency, wavelength, speed).
Concept of relative velocity.
Algebraic manipulation.
Common mistakes
Incorrectly applying sign conventions for source and observer velocities in the formula.
Confusing the speed of the source/observer with the speed of sound.
Assuming the Doppler Effect only applies to sound waves.
Not considering the direction of motion relative to the line joining the source and observer.
Using absolute velocities instead of velocities relative to the medium (if the medium is moving).
Keywords
Doppler Effect
Apparent Frequency
Relative Motion
Sound Waves
Source Velocity
Observer Velocity
Frequency Shift
Redshift
Blueshift
Practice preview
What phenomenon describes the apparent change in frequency of a wave due to relative motion between its source and the observer?…
easy
An ambulance with a stationary siren emits sound of frequency `f`. If an observer moves towards the ambulance with a speed `v_o`, and the speed of sound in air is `v`, what is the apparent frequency `f'` heard by the obs…
easy
A stationary sound source emits a sound of frequency f. An observer moves towards the source with a speed of v_o. If the speed of sound in the medium is v, what is the apparent frequency heard by the observer?…
medium