# Refractive index Query

1. Mar 16, 2006

### 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

:grumpy: what's wrong?

2. Mar 16, 2006

### Staff: Mentor

3. Mar 16, 2006

### 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.

4. Mar 17, 2006

### inha

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

5. Mar 18, 2006

### 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?

6. Mar 19, 2006

### Claude Bile

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

Claude.

7. Mar 20, 2006

### Chaos' lil bro Order

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).

8. Mar 21, 2006

### 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.

9. Mar 21, 2006

### Chaos' lil bro Order

uhhhh so then yes.

10. Mar 22, 2006

Indeed

Claude.