The change in frequency or wavelength of waves from a source as observed by someone moving relative to that source. In astronomy, this causes light from objects moving toward us to appear bluer (higher frequency) and light from objects moving away to appear redder (lower frequency).
Named after Austrian physicist Christian Doppler who described the phenomenon in 1842. Originally applied to sound waves, it was later extended to light waves and became crucial for understanding stellar motion and cosmic expansion.
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