Which statement correctly defines the critical angle in optics?

Which statement correctly defines the critical angle in optics?

• A. The Angle of Incidence for Which Refraction Is 90 Degrees
• B. The Angle of Incidence for Which Reflection Becomes Total
• C. The Angle of Reflection When Light Exits a Medium
• D. The Angle of Incidence at Which Light Slows to Zero Speed

Answer: (A)

The critical angle is the angle of incidence at which the refracted ray in the second medium travels along the boundary, meaning the refracted angle is 90 degrees when light moves from a denser to a rarer medium. This is exactly described by the statement: the angle of incidence for which refraction is 90 degrees. Beyond this angle, light cannot refract and is totally internally reflected. The other descriptions don’t define the critical angle: they either refer to reflection becoming total, or to a reflection angle when exiting, or to light slowing to zero speed (which doesn’t happen in a medium).

The critical angle is the angle of incidence at which the refracted ray in the second medium travels along the boundary, meaning the refracted angle is 90 degrees when light moves from a denser to a rarer medium. This is exactly described by the statement: the angle of incidence for which refraction is 90 degrees. Beyond this angle, light cannot refract and is totally internally reflected. The other descriptions don’t define the critical angle: they either refer to reflection becoming total, or to a reflection angle when exiting, or to light slowing to zero speed (which doesn’t happen in a medium).