Refractive Indices

If a ray of homogeneous light fall on a refracting medium at an angle of incidence φ, the angle of refraction being φ'; then the ratio sin φ/sin φ' is constant for all values of φ, and is the refractive index for light of the given refrangibility going from the first to the second medium.

Let us suppose the first medium is air, then it is not difficult to determine by optical experiments the value of the angle φ, but φ' cannot be determined with any real approach to accuracy. The determination of μ, the refractive index, is therefore generally effected by indirect means. We proceed to describe some of these.⁽¹⁾

(1)	For proofs of the optical formulae which occur in the succeeding sections, we may refer the reader to Glazebrook's Physical Optics, chaps, iv. and viii.