TIA's Glossary of Telecommunication Terms
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critical angle
In geometric optics, at a refractive boundary, the smallest angle of incidence at which total internal reflection occurs. (188) Note 1: The angle of incidence is measured with respect to the normal at the refractive boundary. Note 2: The critical angle is given by 
where 
c is the critical angle, n 1 is the refractive index of the less dense medium, and n 2 is the refractive index of the denser medium. Note 3: The incident ray is in the denser medium. Note 4: If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence.