The critical angle of cornea-air interface is

Critical angle refers to the angle of incidence in the denser medium, corresponding to which angle of refraction in the rare medium is 90deg. At the tear-air interface, the critical angle is approximately 46deg. Made use of in construction of gonio lenses where this interface is changed and as a result critical angle becomes larger allowing for the reflected light to escape the eye and get reflected on the gonio mirrors which helps the examiner visualize the angle of the eye