Doppler Effect
Waves & Sound
38
JEE Qs
5%
Hard
60
min
Master the sign convention and remember that all velocities must be relative to the medium; resolve velocities along the line of sight for angled motions.
🧮 Key Formulas
✅ Key Points for JEE
- 1The Doppler effect describes the change in observed frequency of a wave due to relative motion between the source and the observer along the line joining them.
- 2All velocities (v, v_o, v_s) must be measured relative to the medium in which the sound is travelling. If the medium itself is moving (e.g., wind), its velocity must be accounted for.
- 3Only the component of the source's and observer's velocities along the line joining them contributes to the Doppler shift. Perpendicular components cause a change in direction, not frequency.
- 4Problems involving reflection from a moving surface should be solved in two steps: first, calculate the frequency received by the reflector (treating it as an observer); second, treat the reflector as a new source emitting that received frequency and calculate the frequency heard by the final observer.
⚠️ Common Mistakes
- ✕Incorrect application of sign conventions for observer and source velocities in the formula.
- ✕Forgetting to consider velocities relative to the medium, especially when wind or a moving medium is involved.
- ✕Not using only the component of velocity along the line joining source and observer for calculations when motion is not purely radial.
📝 Practice Questions
See allQ27.Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R Assertion A: A sound wave has higher speed in solids than gases. Reason R: Gases have higher value of Bulk modulus than solids. In the light of the above statements, choose the correct answer from the options given below ⎪ ⎪ 2025 (28 Jan Shift 1) JEE Main Previous Year Paper (1) Both A and R are true but R is NOT the correct (2) A is true but R is false explanation of A (3) A is false but R is true (4) Both A and R are true and R is the correct explanation of A
Q40.Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : The binding energy per nucleon is found to be practically independent of the atomic number A, for nuclei with mass numbers between 30 and 170. Reason (R): Nuclear force is long range. In the light of the above statements, choose the correct answer from the options given below : (1) (A) is true but (R) is false (2) (A) is false but (R) is true (3) Both (A) and (R) are true and (R) is the correct (4) Both (A) and (R) are true but (R) is NOT the explanation of (A) correct explanation of (A)
Q36.A closed organ and an open organ tube are filled by two different gases having same bulk modulus but different densities ρ1 and ρ2′ , respectively. The frequency of 9th harmonic of closed tube is identical with 4th harmonic of open tube. If the length of the closed tube is 10 cm and the density ratio of the gases is ρ1 : ρ2 = 1 : 16, then the length of the open tube is : (1) 15 7 cm (2) 207 cm (3) 15 9 cm (4) 209 cm
Q24.A closed and an open organ pipe have same lengths. If the ratio of frequencies of their seventh overtones is ( a−1a ) then the value of a is ________ → 2^i+6^j+8^k 2^i+^j+^k . The
Q24.Two open organ pipes of lengths 60 cm and 90 cm resonate at 6th and 5th harmonics respectively. The difference of frequencies for the given modes is _______ Hz . (Velocity of sound in air = 333 m/s )
Q2. The equation of stationary wave is : y = 2a sin ( 2πntλ ) cos ( 2πxλ ). Which of the following is NOT correct : (1) The dimensions of n/λ is [T] (2) The dimensions of n is [LT−1] (3) The dimensions of x is [L] (4) The dimensions of nt is [L]
NCERT Chapters
- Class 11 Physics Ch 15: Waves