A wedge of transparent material having first and second angularly displaced faces is positioned in a controllable manner with respect to a right angle prism with one of the faces of the wedge parallel to the hypotenuse face of the prism. A piezoelectric element is positioned to drive the wedge relative to the prism between a first position wherein the wedge and prism are in near optical contact and a second position wherein the wedge and prism are spaced by a distance of about one wavelength of the light incident upon the prism so as to switch the optical path of the incident light beam from a condition of transmission to a condition of transmission to a condition of reflection at the hypotenuse face.