Use the concept of critical angle in terms of the refractive index of a medium with respect to another medium, sinθC=μ1,
⇒μ=sinθC1=sin45∘1=(1/2)1=2, now we know that the ratio of the speed of light in vacuum and speed of light in a medium is called the refractive index of the medium, μ=vc⇒v=μc=23×108ms−1.