The Doppler Effect

Shown by all types of waves

Properties of WavesWaves are characterized by two numbers:

• Wavelength, (size of the wave)

• Frequency, ν (number of waves/second)

For sound waves, frequency more commonly called pitch

For light waves, these are all related by:

c = νWhere c is the speed of light

Longer means Smaller ν

Electromagnetic Wave

MagneticField

ElectricField

The Doppler Effect

Shift in the observed wavelength when the source is moving relative to the observer.

Examples:• Sound Waves (Siren or Train Horn)• Light Waves

Amount of the shift and its sign depends on• relative speed of the source & observer• direction (towards or away)

Stationary Source

Same Pitch Same Pitch

Doppler Effect in Sound

High Pitch(short waves)

Low Pitch(long waves)

The Doppler Effect in Light

Works same as it does for sound

Light moving away from the observer

Wavelength gets longer: REDSHIFT

Light moving towards the observer

Wavelength gets shorter: BLUESHIFT

Way to Measure Speeds

Observe the wavelength (obs) of a source with a known emitted wavelength (em)

The difference is directly proportional to the speed of the source, v:

obs

em

v

c

-

= em

Use on Earth