Fig. 44. A. At anterior corneal surface, rays from chamber angle are incident at greater than critical angle of cornea in air. Result is total internal reflection. B. Contact lens curvature approximately matches cornea. Interspace is filled with fluid (aqueous solution of methylcellulose) whose index is close to index of cornea. Light can then traverse interface because the critical angle has been changed (icrit of cornea in methylcellulose is approximately 90°, therefore almost all rays can pass). (Adapted from Rubin: Optics for Clinicians.)