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Q & A: light as wave

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Most recent answer: 10/22/2007
Q:
What evidence is there for light as a wave? And can you explain the Doppler Effect in the way it relates to light?
- Kimberley
England
A:
There’s an enormous amount of evidence for light as a wave. The most dramatic types of evidence involve interference effects, in which parts of the wave travelling along different paths can recombine to either cancel or enhance each other.
The colors produced by thin films of oil on water are among the countless examples of such interference effects. Different colors of light have different wavelengths. Some have a whole number of waves in a round trip through the oil, and some are halfway between two whole numbers. The interference effects are opposite in those cases, so different colors reflect differently. The effect changes from area to area because the thickness of the film changes.
The Doppler effect for light is similar but not quite identical (due to the importance of special relativity for light) to the Doppler effect for sound. If you bounce light off an object moving away from you, each new wavefront bouncing back toward you will start a little farther behind the previous one than it would have if the object weren’t moving away from you. So the wavefronts will come back to you with a little longer time intervals between them, i.e. at lower frequency. Doppler techniques like this allow us to measure how fast molecules are moving around in solution. Ordinary radar spped guns use a technique like this with lower-frequency electromagnetic waves.

Mike W.

(published on 10/22/2007)

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