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What is the critical angle for a boundary between two materials?
AThe angle at which the reflected ray disappears entirely
BAn incident angle that produces an angle of refraction of 90 degrees
CThe angle at which a ray travels parallel to the boundary before striking it
DThe angle at which two rays of different colour first separate
Answer & Solution
Correct answer: B. An incident angle that produces an angle of refraction of 90 degrees
1. The critical angle is defined as an incident angle that produces an angle of refraction of 90 degrees.
2. Beyond that incident angle, no refracted ray can escape into the second medium at all.
3. It has nothing to do with colour separation, which is dispersion, a different phenomenon entirely.
_Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.2 Refraction_
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