Hello Eswara,

I think you meant snell's law for refraction which states - The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media.

You can derive the law using fermat's principle of light.

Full derivation here - https://byjus.com/snells-law-formula/

Hope this helps.