A straight line SS’, to represent the surface of separation between air and glass, is drawn on a sheet of drawing paper on a drawing-board, together with a normal ON and several lines at various angles to ON to represent incident rays (Fig. 23.2). A ruler is placed along SS’ and a rectangular glass block carefully placed in contact with it in the position shown. The ruler is now transferred to the lower edge of the block and the block is then removed.

A line TT’, to represent the lower edge of the block, is now drawn. Without moving the ruler, the block is now placed carefully in contact with the ruler. The two lines SS’ and TT’ should now coincide exactly with the upper and lower vertical faces of the block.

This is a better method than simply drawing round the block with a pencil. The block is thick and its edges are usually bevelled, which renders it difficult to draw lines to coincide exactly with the block edges. Method. Two pins PI and Pz are stuck in the paper as far apart as possible along one of the lines drawn to represent an incident ray (Fig. 23.3). Then, looking through the block, the emergent ray is located by sticking two sighting pins E; and E2 in the paper exactly in line with the images II and Iz of the pins PI and Pz: This procedure is carried out for all the incident rays, each time marking the positions of the pins with fine pencil crosses. When this has been done the block is removed and the points EI and E2 joined. Finally, the refracted rays are drawn in by joining 0 to the points where the emer gentrays leave the block. The ratio sin i/sin r may now be found for each pair of rays by looking up the sines in tables (at the end of the book). The ratio should be found to be practically constant.

Also, a graph of sin i against sin r should be plotted and a mean value for s~n i sin r found from its gradient. See also the section entitled, Refractive Index. Alternative treatment of results. Instead of measuring the angles i and r and finding the sines from tables, the ratio s~n i may be found by the following method which SIn r Snell himself used. The largest convenient circle centre 0 is drawn and the incident and refracted rays (not the emergent rays) are produced, if necessary, to cut the circle at A and C. Perpendiculars AB and CD are drawn to the normal ON (Fig. 23.4).

Referring to Fig. 23.4 in which, for simplicity, only one pair of rays has been
shown, we see that