What's Wrong with Refractive Index Query? | Wikipedia

AI Thread Summary
The discussion addresses the discrepancy in the refractive index of water as presented by Wikipedia, where the equation n^2 = (Relative Permittivity) x (Relative Permeability) does not hold true when calculated with water's values. The apparent inconsistency arises because the electric permittivity varies with frequency, and the values compared are from different frequency ranges. It is clarified that the refractive index at optical frequencies is accurately determined through measurement, not theoretical calculations. The conversation also confirms that electromagnetic waves, including light, radio waves, x-rays, and gamma-rays, will refract when transitioning between different media, although practical considerations like absorption affect this. Overall, the thread emphasizes the complexities of calculating refractive indices and the behavior of light in various media.
RichRobX
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Wkipedia gives refractive index

n^2 = (Relative Permittivity) x (Relative Permeability)

but taking water as an example
n = 1.33
Relative Permeability = 1.0 (roughly)
Relative Permittivity = 80.4

1.33^2 = 1.77 Which is NOT 80.4


what's wrong?
 
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Keep in mind that the electric permittivity varies with frequency. You are comparing the values for an optical frequency, compared to a very low (A few hundred Hz typically) frequency, hence the apparent discrepancy.

The true value of the electric permittivity at optical frequencies is reliably obtained by measuring the refractive index.

Claude.
 
IIRC we still can't calculate water's refractive index right via electron structure calculations. Water truly is mysterious stuff.
 
Electric fields

Ok thanks that's cleared things up somewhat.

Does this mean that an electric field would be refracted upon going from say air into water?
 
An electromagnetic wave oscillating at optical frequencies (i.e. light) will refract, yes.

Claude.
 
claude,

Would radio waves, x-rays and gamma-rays also refract when entering water (guess I'm assuming no absorbtion which i shouldn't, but still).
 
The law of refraction is derived from conservation of energy and momentum constraints at a boundary between two media with different refractive indices. Nothing in the derivation explicitly forbids refraction above or below a certain wavelength.

These restrictions are placed by more 'practical' considerations such as absorption.

Claude.
 
uhhhh so then yes.
 
  • #10
Indeed

Claude.
 

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