The value of the constant s;in i for a ray passing from one medium to another is called SID r the refractive index of the second medium with respect to the first; and is denoted by the symbol n. If, however, the first medium is air it is usual to speak of n simply as the refractive index of the second medium but, strictly speaking, the absolute refractive index of a medium is the value of n when the first medium is a vacuum.
Thus, if a ray passes from air into water, then It follows that the mean value of the ratio AB obtained in the experiment to verify CD
Snell’s law will be equal to the refractive index of glass. Manufacturers have to know the refractive index of the glass used for aking
lenses of telescopes, microscopes, cameras and other optical instruments. For crown glass, n is about 1.52; for flint glass about 1.65. The refractive index of water is 1.33.