Physics · Class 11

Move a source and an observer

An approaching source compresses the wavefront spacing and raises observed frequency. An approaching observer meets wavefronts faster. Their velocities enter different parts of the sound Doppler formula because the medium defines a preferred frame.

Open the interactive experiment →

Move a source and an observer

ஒலி மூலம் பார்வையாளரை நோக்கி வந்தால் அலைமுனைகளின் இடைவெளி குறைந்து கேட்ட அதிர்வெண் அதிகரிக்கும்.

Key notes, relationships and model limits

Key relationship

fobserved=fsource(c+vobserver)/(c−vsource). Positive velocities are toward the other body.

Remember

Common trap

Source and observer velocities are not interchangeable in the classical sound formula.

Model limits

One-dimensional subsonic motion relative to a stationary uniform medium. Source and observer signs are explicitly toward each other; no wind, relativistic or sonic-shock effects.

What you will learn

Use source and observer signs relative to the sound medium.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

Ahead spacing=(340−40)/500=0.6 m; behind spacing=(340+40)/500=0.76 m. A receding source instead lowers observed frequency.

Why this matters

The model connects move a source and an observer with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

A 500 Hz source approaches at 40 m/s in 340 m/s air; stationary observer hears what?

500 Hz
About 566.7 Hz
441.2 Hz

Read the explanation

fobs=500×340/(340−40)=566.67 Hz.

Course connection: Class XI Chapter 14. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.