Abstract: An optical wave guide used for irradiation purposes in the dental field comprises a portion (10) tapering from a mean diameter at a radiation-entrance end (13) to a relatively small diameter, an intermediate portion (11) of said small diameter and curved about 90.degree., and a portion (12) succeeding the curved portion (11) and flaring out from said small diameter to a relatively large diameter at a radiation exit end (14) of the wave guide. Due to the fact that the curved portion (11) has a relatively small diameter it is possible also to minimize the radius of curvature of this portion without causing radiation loss. At the same time a relatively large area is obtained at the radiation-exit end (14) from which the radiation exits uniformly. The optical wave guide configured in this way is especially suitable for sealing occlusal tooth faces onto which the exit end (14) of the wave guide is flatly placed.