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Stage 2 – Page 9 : Doppler Effect
1️⃣ What is Doppler Effect?
Doppler Effect is the apparent change in frequency (or wavelength) of a wave due to relative motion between the source and the observer.
If source & observer are stationary → no Doppler effect.
2️⃣ Basic Doppler Effect Formula (Sound)
f' = f × ( v ± vo ) / ( v ∓ vs )
- f' = observed frequency
- f = source frequency
- v = speed of sound in medium
- vo = velocity of observer
- vs = velocity of source
3️⃣ Sign Convention (MOST IMPORTANT)
| Case | Sign Used |
|---|---|
| Observer moving towards source | + vo |
| Observer moving away | − vo |
| Source moving towards observer | − vs |
| Source moving away | + vs |
4️⃣ Special Cases (JEE Favorite)
🔹 Case 1: Moving Observer, Source at Rest
f' = f ( v ± vo ) / v
🔹 Case 2: Moving Source, Observer at Rest
f' = f v / ( v ∓ vs )
🔹 Case 3: Both Moving
f' = f ( v ± vo ) / ( v ∓ vs )
5️⃣ Approach vs Recede (Quick Logic)
- Approaching → frequency increases
- Receding → frequency decreases
Always imagine compression or stretching of wavefronts.
6️⃣ Doppler Effect in Still Medium
Medium is assumed stationary (air at rest). Only source & observer motion matters.
7️⃣ JEE-Type Numerical Example
A source of frequency 500 Hz moves towards a stationary observer with speed 20 m/s. Speed of sound = 340 m/s. Find apparent frequency.
Solution:
f' = 500 × 340 / (340 − 20) = 531.25 Hz
8️⃣ Common Mistakes in JEE
- Wrong sign convention
- Using medium velocity incorrectly
- Confusing observer motion with source motion
9️⃣ Conceptual JEE Questions
- Why Doppler effect is not observed in light the same way?
- Effect when wind is present?
- Why beats are not Doppler effect?
➡️ Next Page Preview
- Doppler Effect with Wind
- Relative velocity method
- Advanced numericals
Stage 2 – Page 9 Completed Successfully ✅
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