Doppler Shift For a classical wave, imagine a pond of water on which a periodic disturbance is made. The disturbances generate a series of concentric outward moving pulses (waves). The wavelength is the distance between the pulses and the frequency is the rate at which the pulses pass by an observer.

Doppler Shift For a moving source, the point of periodic disturbance moves (to the right in the diagram). The disturbances still generate a series of outward moving circular pulses (waves), now however on the leading side the peaks are closer to together than on the trailing side. The detected wavelength then depends on the location of the observer. If the source approaches, the wavelength is shortened (blueshift). If the source recedes, the wavelength is lengthened (redshift).

For small speeds, the classical Doppler shift is

• Doppler shift ===> z = speed of recession / speed of light

so that Slipher (and Hubble) noted that Hubble's Law could be taken to mean

• Hubble's Law ===> v = constant x D = Hubble's constant x D = H(now) x D

A modern interpretation of the Hubble Law is that the redshift z is due to an uniform expansion of the Universe and is one of the fundamental observational results for the Universe.

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