Doppler Principle
Historical document, translated for reference. It reflects medical knowledge of the 1920s–30s and is not medical advice.
Summary
The Doppler Principle describes how the perceived wavelength of light or sound depends on the relative motion between the observer and the source. When a sound source approaches the ear, the sound becomes higher in pitch, and when it moves away, the sound becomes lower. Similarly, for light, if a star moves toward Earth, its spectral lines shift toward the blue end of the spectrum, while if it moves away, they shift toward the red end.
Encyclopedia article (1928–1936)
DOPPLER PRINCIPLE (Doppler), the dependence of the perceived wavelength of light or sound on the relative motion between the observer and the source. If a sound source approaches the ear, the sound becomes higher, in the opposite case lower. If a star is moving toward the earth, then the observed thin spectral line will shift toward the blue side of the spectrum; in the case of the star moving away from the earth, the line will shift slightly toward the red side. The Doppler principle is simply explained by the kinematics of wave motion. This can be explained as follows: the observer judges the pitch by the number of impulses reaching his ear per unit of time. If the sound source is approaching, then the number of impulses reaching the observer's ear is greater than with a stationary source—the sound seems higher. Conversely, when the source is moving away, the number of impulses decreases—the sound seems lower. A similar line of reasoning can be applied to light as well.
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“Doppler Principle.” Soviet Medical Encyclopedia. English translation of Bolshaya Meditsinskaya Entsiklopediya, 1st ed. (Moscow, 1928–1936), ed. N. A. Semashko. https://sovietmedicalencyclopedia.pages.dev/article/doppler-principle/