There are different sizes of rectangular glass prisms although for national exams the specific dimensions of the glass prisms are given. The refractive indices also range from around 1.4 to 1.6 depending with the material used.

1. The pins should be properly placed by avoiding any parallax error. 2. The glass block outline should be properly traced and when replacing the block, you should confirm that all the four sides are properly placed. 3. Measure the angle of incidences and refraction accurately. 4. While observing the pins, move your eyes across to confirm the best position of the alignment.