ray incident from the entrance surface to the exit surface and a groove of v shaped cross section disposed in the boundary between the entrance surface and the exit surface application_number US 74494091 A description the present invention will hereinafter be described in detail with respect to an embodiment thereof shown in the drawings fig3 is a cross sectional view showing the state of light rays in the rectangular prism of the present invention in the construction shown an entrance surface 10 and an exit surface 20 lie in the same plane and cooperate with reflecting surfaces 30 and 40 orthogonal to each other to form a substantially right angled isosceles triangle a light ray 2 incident from the entrance surface 10 and which and which is indicated by broken line is reflected by the first reflecting surface 30 and is totally reflected by the inclined surface 50 of a v groove which is adjacent to the entrance surface 10 therefore the beam 2 does not emerge from the exit surface 20 however there is left the possibility of the totally reflected ghost light being repetitively reflected in the prism and emerging from the exit surface 20 so in the present invention in order to prevent this the angle a of the inclined surface of the v groove with respect to the entrance surface 10 is determined as per the following conditional expression.