Day 40 – Morning Session: Physics – Doppler Effect – ECET 2026

The Doppler Effect is one of the most important and scoring topics in ECET Physics. Questions are usually asked on formulas for apparent frequency, direction of motion, and real-life applications (sound, light, radar, etc.).

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📘 Concept Notes

🌐 What is Doppler Effect?

• The Doppler Effect is the change in frequency (or wavelength) of a wave as observed by an observer due to the relative motion between the source and the observer.
• Example: Ambulance siren sounding higher when coming towards you and lower when going away.

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⚙️ Formulas (QuickLaTeX format)

1. General Doppler Effect Formula (Sound):

Where:

• = apparent frequency
• = actual frequency of source
• = velocity of sound in medium
• = velocity of observer (positive if moving towards source)
• = velocity of source (positive if moving away from observer)

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2. Case – Source Moving Towards Stationary Observer:

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3. Case – Observer Moving Towards Stationary Source:

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4. Case – Both Moving Towards Each Other:

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5. Case – Both Moving Away from Each Other:

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📐 Example Problem

An ambulance siren emits sound of frequency . It moves towards a stationary observer with velocity . Speed of sound in air .

Apparent frequency:

So, the observer hears a higher pitch (637.5 Hz).

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🔋 Applications of Doppler Effect

• Police radars (speed detection).
• Astronomy (measuring star/galaxy velocities).
• Weather forecasting (Doppler radar).
• Medical imaging (Doppler ultrasound).
• Sirens of vehicles (ambulance, trains).

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🔟 10 Expected MCQs – ECET 2026

Q1. Doppler Effect refers to:
A) Change in amplitude due to motion
B) Change in frequency due to relative motion
C) Change in velocity of wave
D) Change in medium

Q2. Apparent frequency formula is:
A)
B)
C)
D)

Q3. If source moves towards observer, apparent frequency:
A) Increases
B) Decreases
C) Same
D) Zero

Q4. In Doppler formula, is:
A) Speed of wave
B) Velocity of observer
C) Velocity of source
D) Apparent frequency

Q5. An ambulance of frequency moves towards an observer with . . Apparent frequency is?
A) 530 Hz
B) 550 Hz
C) 580 Hz
D) 600 Hz

Q6. If both source and observer move away from each other:
A) Apparent frequency increases
B) Apparent frequency decreases
C) Apparent frequency same
D) None

Q7. Which of these is NOT an application of Doppler Effect?
A) Astronomy
B) Radar
C) Seismograph
D) Medical ultrasound

Q8. If source is stationary and observer moves towards source:
A) Frequency decreases
B) Frequency increases
C) Frequency same
D) Zero

Q9. Which wave shows Doppler Effect?
A) Sound only
B) Light only
C) Both sound and electromagnetic
D) None

Q10. The pitch of a siren appears higher when:
A) Source moves away
B) Source stationary
C) Source moves towards observer
D) Observer moves away

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✅ Answer Key

Q.No	Answer
Q1	B
Q2	A
Q3	A
Q4	B
Q5	B
Q6	B
Q7	C
Q8	B
Q9	C
Q10	C

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🧠 Explanations

• Q1 → B: Doppler Effect = change in frequency due to motion.
• Q2 → A: General formula = .
• Q3 → A: Approaching → higher frequency.
• Q4 → B: is velocity of observer.
• Q5 → B: .
• Q6 → B: Moving away → frequency decreases.
• Q7 → C: Seismograph is not Doppler-based.
• Q8 → B: Moving towards → frequency increases.
• Q9 → C: Both sound and light show Doppler Effect.
• Q10 → C: Approaching source increases pitch.

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🎯 Why Practice Matters

• Doppler Effect problems are direct formula-based and scoring.
• Frequently repeated in ECET exams.
• Also used in real-world tech like radars and astronomy → helps in interviews.

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