Which set of conditions is necessary for total internal reflection to occur at an interface? 
ALight must travel from a rarer medium to a denser medium, and the angle of incidence must be less than the critical angle.
BLight must travel from a denser medium to a rarer medium, and the angle of incidence must exceed the critical angle.
CLight must travel from air to glass, and the angle of refraction must be $90^\circ$.
DLight must strike the interface normally from any medium.
Answer & Solution
Correct answer: B. Light must travel from a denser medium to a rarer medium, and the angle of incidence must exceed the critical angle.
Total internal reflection occurs only when light goes from an optically denser medium to a rarer medium. As the angle of incidence increases, the refracted angle increases until it reaches $90^\circ$ at the critical angle; beyond this, refraction is impossible and the ray is totally reflected.
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