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

μ2μ1=v1v2=12⇒μ1μ2=2μ1>μ2For total internal reflection  2μ1=1sinC⇒μ1μ2=1sinC⇒2=1sinC⇒C=30oSo, for total (Internal reflection angle of incidence must be greater than 30°).