What's Wrong with Refractive Index Query? | Wikipedia

[RichRobX]

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?

 

[Doc Al]

Check out this thread for an explanation: https://www.physicsforums.com/showthread.php?t=104510

 

[Claude Bile]

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.

 

[inha]

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

 

[RichRobX]

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?

 

[Claude Bile]

An electromagnetic wave oscillating at optical frequencies (i.e. light) will refract, yes.

Claude.

 

[Chaos' lil bro Order]

claude,

Would radio waves, x-rays and gamma-rays also refract when entering water (guess I'm assuming no absorption which i shouldn't, but still).

 

[Claude Bile]

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.

 

[Chaos' lil bro Order]

uhhhh so then yes.

 

[Claude Bile]

Indeed

Claude.