Magazine article UNESCO Courier

Red Shift and the Doppler Effect

Magazine article UNESCO Courier

Red Shift and the Doppler Effect

Article excerpt

Like Monsieur Jourdain, the hero of Moliere's Le Bourgeois Gentilhomme, to whom it came as a considerable shock to learn that he had been speaking prose all his life, anyone who has stood on a railway station platform as an express train thunders past with whistle blowing, may be surprised to learn that he or she is quite familiar with the "Doppler effect", one of the keys to modern cosmology.

When a train driver blows his whistle as he approaches a station, he hears a steady sound at a fixed pitch. To the man standing on the platform, however, the sound of the whistle rises in pitch as the train approaches and drops again as the train spees away into the distance. This is because sound is a succession of waves in the air. As a source of sound approaches, the waves are compressed and the pitch rises, as it recedes, the waves are stretched out and the pitch falls.

The Doppler effect is named after the 19th-century Austrain physicist Christian Doppler who pointed out that the effect applies also to light waves. …

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