It's just obeying the law of light/whatever reflection.
n¹ * sine(ø¹) = n° * sine (ø)
n¹ * sine(øcrit) = nr * sine (90 degrees)
n¹ * sine(øcrit) = n°
sine(øcrit) = n°/n¹°
øcrit = sine-1 (n°/n¹) = invsine (n°/n¹)
I better stop pretending I know what I'm talking about. What is this? Advanced maths? I hate maths.
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