Doppler EffectDoppler Effect

Moving SourceMoving Source

Light moves equally in all directions from a source.Light moves equally in all directions from a source.• Circular (or spherical) patternCircular (or spherical) pattern

If the source is moving the origin of successive circles If the source is moving the origin of successive circles moves.moves.

Wavelength ShiftWavelength Shift The normal period is The normal period is TT and the and the

observer moves at speed observer moves at speed uu..

The wavelength ahead of the source The wavelength ahead of the source is shorter.is shorter.• ’’= = - - uTuT

The wavelength behind the source is The wavelength behind the source is longer.longer.• ’’= = + + uTuT

Frequency ShiftFrequency Shift The period is related to the The period is related to the

wavelength.wavelength.

This can be converted into a This can be converted into a frequency shift.frequency shift.• Requires correction for special Requires correction for special

relativityrelativity cv

cvff

fv

/1

/1

/

Moving ObserverMoving Observer

If the observer is moving toward the source the effect is If the observer is moving toward the source the effect is similar, but the wave crests have a different relative similar, but the wave crests have a different relative spacing.spacing.

)/1( vuff A speed gun uses a double Doppler shift. There is a shift for both the pulse out and reflected signal.

Speed GunSpeed Gun

A speed gun used the doppler shift twice.A speed gun used the doppler shift twice.• Shift for pulse out and pulse backShift for pulse out and pulse back