About the Doppler Effect
The
Doppler effect was named after Christian Doppler, who first came up with the
idea in 1842. He thought that the frequency of sound waves would change if
either the source or the observer was moving. If they were approaching, the
frequency would be higher; if they were diverging, the frequency could be
lower.
There are many everyday examples of the Doppler effect: train whistles, police and fire sirens, race car engines. When a train is approaching, the whistle has a higher pitch than normal. You can hear the change in pitch as the train passes. After the train has passed, the pitch should be lower than normal. The same is true with sirens on police cars and the engines of race cars.
One
way to visualize the Doppler effect is to think of sound waves as pulses emitted
at regular intervals. Imagine walking forward. Each time you take a step, you
emit a pulse. Each pulse in front of you would be be a step closer together than
if you were emitting them while standing still; each pulse behind you would be a
set further apart. in other words, the pulses in front of you have a higher
frequency than at rest and the pulses behind you have a lower frequency at rest.
The Doppler effect doesn't just apply to sound. It works with all types of waves. This includes light. Edwin Hubble used the Doppler effect to determine that the universe is expanding. Hubble saw that light from other galaxies had a higher frequency than it should. He found that the light emitted by the galaxies was shifted toward higher frequencies, or the red end of the spectrum. This is know as a red Doppler-shift, or a red-shift. If the galaxies were moving toward Hubble, the light he observed would have been blue-shifted.
Jeff Danger, Science
Ranger
Cool Science Show For Kids