Doppler effect…

“Late Lament”

Breathe deep the gathering gloom, Watch lights fade from every room. Bedsitter people look back and lament, Another day’s useless and idious spent. Impassioned lovers wrestle as one, Lonely man cries for love and has none. New mother picks up and suckles her son, Senior citizens wish they were young. Cold hearted orb that rules the night, Removes the colours from our sight. Red is grey and yellow white. But we decide which is right. And which is an illusion? (Moody Blues Lyric)

The Doppler effect (or Doppler shift), named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague, is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

The relative changes in frequency can be explained as follows. When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore each wave takes slightly less time to reach the observer than the previous wave. Therefore the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are travelling, the distance between successive wave fronts is reduced; so the waves “bunch together”. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is increased, so the waves “spread out”.

ARE YOU WATCHING CLOSELY…
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