Light wave enters from medium 1 to medium 2. Its velocity in 2nd medium is double from 1st. For total internal reflection the angle of incidence must be greater than

μ2μ1=v1v2=12⇒μ1μ2=2μ1>μ2  For total internal reflection 2μ1=1sin⁡C⇒μ1μ2 =1sin⁡C⇒2=1sin⁡C⇒C=30∘ So, for total Internal reflection angle of incidence must be greater than 30°.