the angle of incidence where a ray of light which is traveling from an optically denser to rarer medium is refracted at 90° to the normal (i.e., it skims the surface of the two media in contact).
engela kwesifo lapho wokukhanya ukukhanya kwelanga besuka i optically luthe ukuqina ukuze medium lingandile is refracted at 90 ° evamile (ie, it Eziko ebusweni abezindaba ezimbili in contact).