# Why no transparent materials with large refractive index?

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1. Dec 18, 2014

### jfizzix

I know that the refractive index is determined by a material's dielectric constant and magnetic permeability.

It's also true that we can treat the refractive index as a complex function with the imaginary part giving you an absorption spectrum.

You can then get the index of refraction from the absorption spectrum with the Kramers-Kronig relations.

My question is, what makes it that there are so few if any transparent materials with an index of refraction greater than 4?

2. Dec 18, 2014

Staff Emeritus
How many materials irrespective of transparancy have an index that high?

3. Dec 18, 2014

### jfizzix

Silicon at 500 nm has a refractive index of 4.3 or so. Coming up with more specific examples would be a challenge, but there's a whole field of slow light optics for narrowband light sources tuned to particular spectral lines, say of Rubidium vapor

4. Dec 18, 2014

### jfizzix

Long story short, there are lots of materials that have a high index at one wavelength or another, but not that also are transparent. I don't know why you can't have both, though

5. Dec 19, 2014

### DrDu

The index of refraction averaged over some frequency range is seriously restricted by sum rules. This does not preclude n to become very large near a resonance. However, near a resonance, you will also get absorption. The absorption follows a Lorentzian line shape, hence the imaginary part decays like $1/(\Delta f)^2$ with distance from the center of the line. On the other hand, the real part will decay like $1/(\Delta f)$ So for suffiently narrow absorption lines, you can get very close to the center of the line without getting appreciable absorption but a high index of refraction. In solid state this requires low temperatures to reduce spectral broadening.