# Calculating refractive index

1. Mar 24, 2012

### Francy

How can i calculate the refractive index knowing only the frequency of the light entering the medium from air?

n(refractive index) = (speed of light in vacuum) / (speed of light in medium)

2. Mar 25, 2012

### Redbelly98

Staff Emeritus
You can't, unless you are provided with a formula for the refractive index in terms of frequency or wavelength.

p.s. Welcome to Physics Forums.

Last edited: Mar 25, 2012
3. Mar 25, 2012

### Francy

Yeah, I think the problem I have here doesn't have all the information. Thanks anyway! Thanks also for welcoming me here ^^

4. Mar 25, 2012

### Bob S

I believe that for real indexes of refraction, Re (n(λ)) = c/v(λ).

If the index of refraction varies with wavelength (dispersion), then there is absorption as well as dispersion, which is equivalent to a complex index of refraction. See http://en.wikipedia.org/wiki/Refractive_index
See in particular the section on Complex index of refraction and absorption.

5. Mar 25, 2012

### Redbelly98

Staff Emeritus
But in this situation we don't have v(λ), we only have the frequency $\nu$. From that we can get λ in air, but not λ in the medium or v(λ) for that matter.

6. Mar 25, 2012

### Bob S

I should have said for ANY n(λ), the velocity v(λ) = c/Re (n(λ))