μ=sinθc\mu=\sin \theta_{c}μ=sinθc
μ2=sinθc\dfrac{\mu}{2}=\sin \theta_{c}2μ=sinθc
μ=1sinθc\mu=\dfrac{1}{\sin \theta_{c}}μ=sinθc1
sinθc=2μ\sin \theta_{c}=2 \musinθc=2μ