The easiest demonstration of the Doppler effect uses sound waves. When a noise approaches a stationary observer, the pitch of the sound (the frequency) increases. As the source passes, the pitch decreases as it speeds away from the observer. This effect applies to all forms of waves, including light. Stars are all made up of mostly hydrogen and spectral analysis shows this to be true, but the spectral lines are moved uniformly towards the red end of the spectrum, or lower frequencies. This suggests the stars are moving away from our solar system and is part of the evidence that the universe is expanding.
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