Aim: Finding the refractive index of a prism.
Material required: Prism, piece of white chart of size 20x20 cm, pencil, pins, scale and protractor.
Procedure: Take a prism and place it on the white chart in such a way that the triangular base of the prism is on the chart. Draw a line around the prism (boundry) using a pencil. Remove the prism.
• What is the shape of the outline drawn?
It is a triangle. Name its vertices as P,Q, and R.[for many prisms the triangle formed is equilateral]. The refracting surfaces could be rectangular in shape. Find the angle between PQ and PR . This is the angle of the prism (A).
Mark M on the side of triangle PQ and also draw a perpendicular to PQ at M. Place the centre of the protractor at M and along the normal. Mark an angle of 300 and then draw a line up to M. This line denotes the incident ray. This angle is called angle of incidence. Note it in a table (1). Draw a small arrow on it as shown in figure 8

Place the prism in its position (triangle) again. Now fix two pins vertically on the line at points A and B as shown in figure 8. Look for the images of pins through the prism from the other side (PR) and fix another two pins at points C and D in such a way that all the four pins appear to lie along the same straight line. Do it carefully. Now remove the prism and take out pins. Draw a line joining the two pin-holes formed by the pins to meet surface ‘PR’, this is the emergent ray which ‘emerges from’ the surface PR at a point ‘N’. The angle between the normal at N and the emergent ray is the angle of emergence. Measure this angle and note its value in the table (1)