Is it just that the medium becoming less dense upon increasing the temperature allows the light to travel faster? I am trying to explain using the polarisation concept. Means the refractive index is the square root of relative permittivity, so can we say the temperature changes causes changes in polarization due to increase in average expansion of charges, hence the dipole moment and thus the polarization?
Refarctive index is square root from dielectric constant. There are a few reasons which influence on dielectric constant: virtual interband electron transitions, phonons and free carriers. To answer on your question it is necessary to know what matter do you mean and which spectral range.
I did an experiment to determine the refractive index of certain liquids and also see their temperature dependence of refractive index, using Abbe refractometer. The refractive index of the prism was ~1.75. and the liquids under investigation were benzene, ethanol, water . I did found a change in the refractive index with increasing temperature.
As implied above, the temperature dependance of the index of refraction (dn/dT) depends on many factors. For example two common glasses have dn/dT of 3.4 for BK7 and 10 for SiO2.
One can find a good introduction at Wikipedia under "refractive index"
However, an excellent discussion of all of the wondrous complications and dependences of dn/dT can be found at:
http://photonics.intec.ugent.be/education/IVPV/res_handbook/v2ch33.pdf
Temperature is one of the main factors which affect the refractive index. The book given by Douglas is a very useful source. For example please read the p.27 in the book which discusses the thermal expansion of solids.
For glasses the change of density with temperature is not the only reason for change of refractive index because of the behaviour of so called athermal glasses, which are showing a decreasing refractive index with increasing temperature (e.g. see glass N-PK51 on graph on side 6 at following technical information of SCHOTT)
http://www.schott.com/advanced_optics/german/download/schott-tie-29-refractive-index-and-dispersion-april-2014-eng.pdf
Dear Ms. Fatemeh Makouei
Al salamu alikom
The attached articles may help you.
My regards
https://www.researchgate.net/publication/294426798_Studies_on_dielectric_properties_opto-electrical_parameters_and_electronic_polarizability_of_thermally_evaporated_amorphous_Cd50S50-xSex_thin_films
https://www.researchgate.net/publication/304150361_Study_of_optical_properties_of_thermally_evaporated_ZnSe_thin_films_annealed_at_different_pulsed_laser_powers
Article Studies on dielectric properties, opto-electrical parameters...
Article Study of optical properties of thermally evaporated ZnSe thi...
Temperature is one of the main factors influencing the refractive index of solids . The thermo-optical coefficients dn /dT (or dE/dT) can be estimated from a derivation of the Clausius-Mossotti relationship
In a physical or chemical view, in a homologous series of liquids and/or solutions, the refractive index can be depended on the power of interaction between molecules. The molecules move faster when temperature increases and in this condition, the power of interaction between molecules becomes less.
Well refractive index does varying however, it doesn't directly kindred with temperature nevertheless, it changes because of a change in density. Increased temperature usually reduces density because most materials expand when heated (but some contract). If density is kept the same while temperature changes for instance : by varying applied pressure, then there is no change in refractive index.
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