C.5 Doppler effectIB Physics SL: Revision notes
Section 1
What the Doppler effect is
The Doppler effect is the change in observed frequency (and wavelength) of a wave when there is relative motion between the source and the observer. Approaching gives a higher frequency; receding gives a lower frequency. It happens for sound (and other mechanical waves) and for electromagnetic waves such as light. For sound, the speed of the wave is fixed by the medium, so it matters whether the source or the observer is moving. Light needs no medium, so only the relative velocity matters.
The source's own frequency never changes. It is the frequency received that changes.
Section 2
Wavefront diagrams: moving source
A stationary source produces concentric circular wavefronts, evenly spaced. When the source moves, each wavefront is emitted from a new position, so the circles are no longer concentric: they are crowded together in front of the source (shorter wavelength, higher frequency) and spread out behind it (longer wavelength, lower frequency). The wave speed through the medium is unchanged.
When describing a wavefront diagram in words, say where the circles are centred and where they are closer together.
Section 3
Wavefront diagrams: moving observer
If the source is stationary and the observer moves, the wavefronts stay concentric and evenly spaced, so the wavelength is unchanged. An observer moving towards the source meets the wavefronts at a greater relative speed, so receives more wavefronts per second (higher frequency); moving away, fewer per second (lower frequency).
Section 4
Doppler effect for light
When the relative speed v is much smaller than c:
A source moving away gives an increase in wavelength, a redshift; a source approaching gives a blueshift. The same equation applies to radar and speed guns, but a wave reflected from a moving object is shifted twice, so Δf ≈ 2fv/c.
Only use Δλ/λ ≈ v/c when v ≪ c.
Section 5
Spectral lines and motion in space
Atoms emit and absorb light at characteristic wavelengths, the spectral lines. In the spectrum of a star or galaxy, the pattern of lines identifies the elements, and a uniform fractional shift of every line reveals the line-of-sight velocity. Distant galaxies show redshifts, so they are receding. A star in a binary system shows lines that shift back and forth periodically as it orbits. Rotating stars and galaxies show one side blueshifted and the other redshifted. Only the component of velocity along the line of sight can be measured.
Every line shifts by the same fraction Δλ/λ, not by the same number of nanometres.
That's the notes covered.
Carry on to the next subtopic.