Edexcel A Level (IAL) Physics-5.45 Doppler Shift- Study Notes- New Syllabus

Doppler Effect: Frequency Shift Due to Relative Motion

The Doppler effect describes how the observed frequency of a wave changes when there is relative motion between the source of waves and the observer.

What Is the Doppler Effect?

The Doppler effect occurs when:

• A wave source moves relative to an observer, or
• An observer moves relative to a wave source.

This relative motion causes the observed frequency to differ from the emitted frequency.

Source Moving Towards the Observer

• Wavefronts are compressed.
• Wavelength decreases.
• Observed frequency increases.

Result:

• Higher pitch for sound.
• Blueshift for light.

Source Moving Away from the Observer

• Wavefronts are stretched.
• Wavelength increases.
• Observed frequency decreases.

Result:

• Lower pitch for sound.
• Redshift for light.

 Doppler Effect for Waves

The Doppler effect changes:

• Observed frequency
• Observed wavelength

It does NOT change:

• The speed of the wave in the medium

 Simple Doppler Effect Equation (Source Moving, Observer Stationary)

Approaching source:

\( f’ = f \dfrac{v}{v – v_s} \)

Receding source:

\( f’ = f \dfrac{v}{v + v_s} \)

• \( f’ \) = observed frequency
• \( f \) = emitted frequency
• \( v \) = wave speed
• \( v_s \) = speed of source

Doppler Effect for Light (Qualitative)

• Approaching source → blueshift
• Receding source → redshift
• Used in astronomy to study star and galaxy motion

Important:

• Relativistic equations are used at very high speeds.
• For IAL Physics, qualitative understanding is sufficient.