REFLECTING PRISM. Suppose F GH to be a glass prism having equal sides FG, GH, and the angle at G a right angle ; and let AB be a ray of light incident at B perpendicularly to the side GH. This ray vrill not suffer deviation on entering the glass, but will proceed in the same straight line till it comes to C. What will then happen to it ? Draw Cn at right angles to FH, and make the angle nCo equal to the " critical angle " of the glass. (See " Refraction.") This angle will lie between 39°, and 42°, ac cording to the refractive index of the glass, being the least for flint and the greatest for crown glass. Therefore all rays within the glass incident at C and not lying within the angle neo will suffer total internal reflexion. Now the angle nCB=45', and is there fore greater than nCo, consequently the ray B C is internally reflected at C, and follows the rectilinear course CDE, CE being at right angles to CA.
It appears therefore that the back of a glass prism may be used as a reflector.
With respect to oblique rays incident at B. Tt is evident that all rays incident at B and lying within the angle ABH will suffer inter nal reflexion, but between A and G there will be a limit, for a ray PB whose direction BQ within the glass makes an angle BQH greater than oCH, will nut suffer total internal reflexion, but will pass through the prism in the direction QR. In order for this to happen, the angle P B A must be greater than about 9° when the prism is made of dense ffint glass. All rays therefore lying within a space PH, nearly equal t,o 100°, suffer internal reflexion ; and those lying within PG pass through the prism.