# Refractive index

by Christa
Tags: index, refractive
 P: 2 This is from my Physics practical experiments. The experiment is to find the refractive power of the prism for different colours and also to determine the dispersive power of the material of the prism. My question is how does the refractive index of the material of the prism vary with the wavelength of the source light used? The answer is the refractive index decreases with the increase in the wavelength of the light. I do not want the equation of the relation but the physical significance of the relation.
 PF Patron Sci Advisor Thanks Emeritus P: 38,429 If this is for "practical experiments", then DO IT and find out! Measure the angle of refraction of several different kinds of transparent materials and wavelengths. For a "practical experiment" you shouldn't be trying to find a formula before doing the experiment.
 Sci Advisor P: 1,433 You are unlikely to find such an equation anyway because it is dependant on the composition of the glass. So unless you know the composition of the glass exactly (which I very much doubt), the only way to obtain this information is to measure it directly anyway. Claude.
P: 2

## Refractive index

The answer is the refractive index decreases with the increase in the wavelength of the light.
I do not want the equation of the relation but the physical significance of the relation.
P: 1,433
 Quote by Christa I do not want the equation of the relation but the physical significance of the relation.
This is a pretty open ended question - I would start with the fact that different wavelengths will travel through the glass at different speeds, which results in material dispersion - different wavelengths separate in time and space when propagating through the glass.

Claude.
 P: 4 You can find dispersion curves (refractive index as a function of wavelength) for many materials on RefractiveIndex.INFO
PF Patron
P: 347
 Quote by Christa The answer is the refractive index decreases with the increase in the wavelength of the light. I do not want the equation of the relation but the physical significance of the relation.
The refractive index is related to the velocity of light in the medium.
The velocity of light in the medium is related to the interactions of the incoming electric
field of the radiation ,with the electric fields in the medium.
The higher the frequency of the radiation , the more radiation/medium electric field interactions, which reduces velocity and equals a higher refractive index.
P: 25
 Quote by mpolyanskiy You can find dispersion curves (refractive index as a function of wavelength) for many materials on RefractiveIndex.INFO
Is that your website? It's great! Very useful :-)
P: 4
 Quote by lichen Is that your website? It's great! Very useful :-)
Thanks!

You can check also an advanced version of the site: Dispersion.INFO - Chromatic dispersion
P: 5,408
 Quote by Christa My question is how does the refractive index of the material of the prism vary with the wavelength of the source light used? The answer is the refractive index decreases with the increase in the wavelength of the light. I do not want the equation of the relation but the physical significance of the relation.
That is true for normal dispersion: the refractive index decreases as the wavelength increases. Until an resonance peak, that is- then there is anomolous dispersion, where the refractive index *increases* with increasing wavelength. Anomolous dispersion is responsible for those "light moving faster than 'c'" experiments.

The phenomenon of resonance is the origin of normal dispersion.
PF Patron
P: 347
 Quote by Christa The answer is the refractive index decreases with the increase in the wavelength of the light. I do not want the equation of the relation but the physical significance of the relation.
The following is a more complete explanation:
At the microscale an electromagnetic wave is slowed in a material because the electric field
creates a disturbance in the charges of each atom- primarily the electron proportional to the permittivity.
The oscillation of charges itself causes the radiation of an electromagnetic wave that is slightly out of phase with the original. The sum of the two waves creates a wave of the same frequency but shorter wavelength than the original leading to slowing of the waves travel. (1)
An electron in an atom or molecule is bound there by strong restoring forces. It has a definite natural frequency. For electrons in atoms it is usually in a region corresponding to violet or U.V. light.
In mechanical systems it is possible to "drive" the system most effectively if we impress on it an external force whose frequency is as close as possible to that of the natural resonant frequency.
In the case of light the blue is closer to the natural resonant frequency of the bound electrons than red light. Therefore we would expect the blue light to be more effective in causing the electrons to oscillate. (2)
ref. :
1) http://explanation-guide.info
2)Physics Halliday-Resnick p 1077
 P: 25 Another link to a good site I've never seen before. This thread is a gold mine for good sites.

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