CBSE Class 10 Answered

We know that, refractive index = sin i / sin r

Now, let us consider a ray of light incident at an angle i_c ( critical angle) from glass to air, for which the angle of refraction is 90⁰.

So, the refractive index of air w.r.t. glass = sin i_c  / sin 90⁰.

but, sin 90⁰ = 1

so, the refractive index of air w.r.t. glass = sin i_c  / 1

or, the refractive index of glass w.r.t air = 1/ sin i_c 

so, knowing the refractive index of the denser medium w.r.t. rarer medium ,

we can calculate the critical angle for that pair of media or vice-versa.