![]() The comparison spectrum of an element on Earth, at rest compared with the observer, is shown above and below each galactic spectrum.įor very high speeds the simple formula cannot be used and the effects of special relativity have to be allowed for. Velocity of galaxy (v) = Δλc/λ where λ is the wavelength of a line in the spectrum on Earth, Δλ is the shift in wavelength and c is the speed of light.Ī diagrammatic version of the shift of two absorption lines for three galaxies together with their speeds of recession is shown in the following diagram. The speed (v) relative to the observer on the Earth is given by: If the Doppler shift of lines within their spectra can be measured their speed of recession can be calculated. ![]() This shift towards the red is called the Red Shift and is very good evidence for the expansion of the Universe and for the origin of the Universe in the Big Bang. Observations of the spectra of galaxies show that the light coming from many of these is shifted significantly towards the red and this shows that they are moving away from us at high speeds, many tens of thousands of kilometres per second. The Doppler Effect predicts that radiation from sources that are towards us will be shifted towards shorter wavelengths (the blue end of the spectrum in the case of visible light) and towards longer wavelengths (the red end of the spectrum) if they are moving away from us. ![]() ![]() Possibly one of the most important uses of the Doppler Effect has been the measurement of the motion of galaxies. ![]()
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