Refractive Index: We know that when light passes obliquely from one medium to another, it changes direction in the second medium. The extent to which change in direction takes place in the given set of a medium is termed as refractive index. Refraction Of Light Consider a ray of light passing from medium 1 to medium 2 as shown in fig. v1= speed of light in medium 1 v2= speed of light in medium 2 The refractive index of medium 2 with respect to 1 can be written as below: