Doppler — Observer Moving
Observer's motion changes perceived frequency.
Key Notes
Observer moving, source stationary: apparent frequency changes.
Observer moving TOWARD source: encounters more waves per second ⇒ f' > f.
Observer moving AWAY from source: fewer waves per second ⇒ f' < f.
Formula: f' = f × (v ± v_o)/v. Plus when observer approaches.
v_o = observer speed; v = wave speed.
Same overall direction of shift as moving source, but DIFFERENT formula.
Magnitude depends on RATIO v_o/v.
Combined source + observer motion uses both numerator AND denominator changes.
Formulas
Observer moves, source at rest
Plus when observer approaches (higher f'); minus when receding.
Combined formula
Top: observer; bottom: source. Both signs chosen per geometry.
Important Points
APPROACHING observer: f' > f. RECEDING observer: f' < f.
Source and observer motion give different formulas — important not to confuse.
Sign convention: observer or source MOTION TOWARD adds to numerator (observer) or subtracts from denominator (source).
If both move with same velocity (e.g., car following car): no Doppler shift.
Doppler effect for sound: speeds < 100 m/s ⇒ effect ~30% on f.
For light (no medium), only relativistic Doppler applies: f' = f·√((1−β)/(1+β)) for receding.
Doppler — Observer Moving notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.