MadSci Network: Physics
Query:

Re: why do different wavelengths refract differently?

Date: Tue Apr 20 23:50:06 1999
Posted By: Steven Levin, Staff, Astrophysics, Jet Propulsion Laboratory
Area of science: Physics
ID: 924233242.Ph
Message:

Different wavelengths/frequencies refract differently because the
speed of light in the material (and thus the index of refraction)
is a function of frequency.  For most materials, the index of
refraction is higher for purple light than for red light.  Thus
shorter wavelengths usually refract more, when dealing with visible
light.  At other frequencies this is not always the case.  

For example, when seen in far-infrared light, water has an index
of refraction which gets bigger at lower frequencies.  Thus,very
low frequency infrared light refracting through water will bend 
more towards the normal than high frequency infrared light.
At frequencies of visible light, water's index of refraction is
slightly higher for higher frequencies, so blue or purple light
refracts more than red light.

How the index varies with frequency just depends on the material
involved, and on how its molecules vibrate in response to different
frequencies of light.  High frequencies try to vibrate the molecules
rapidly, and low frequencies try to vibrate them slowly. The response
is different, so the index of refraction is different. 



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