What defines the critical angle in optics?

The critical angle is defined as the angle of incidence at which light traveling from a denser medium to a less dense medium is refracted at an angle of 90 degrees to the normal. At this angle, the refracted ray travels along the boundary between the two media. This phenomenon occurs because as the angle of incidence increases, the angle of refraction also increases; when the angle of refraction reaches 90 degrees, any further increase in the angle of incidence results in total internal reflection rather than refraction. Understanding the critical angle is crucial in applications like fiber optics and in understanding how light behaves at the interface of different media.

In this context, the other options do not accurately describe the critical angle. The maximum angle of incidence that produces no refraction refers to a different concept and is not specifically the critical angle. The angle at which light enters a medium does not address the transition between media in terms of critical angle, while the angle of refraction in a denser medium is not related to the critical angle, which specifically involves the transition from denser to less dense mediums.