PRACTICE / Total internal reflection

Which angle separates refraction from total reflection?

Light travels from a transparent medium of refractive index 1.25 into air of index 1.00.

What is the critical angle, measured from the normal inside the medium?

1. A$\sin^{-1}(5/4)$
2. B$\cos^{-1}(4/5)$
3. C$90^\circ$
4. D$\sin^{-1}(4/5)$

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