Aim : Finding the refractive index of the prism.
Material required : Prism, piece of white chart of size 20 x 20 cm, pencil, pins, scale and protractor.
1) 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.
2) Draw a line around the prism using a pencil. Remove the prism.
3) It is a triangle. Name its vertices P, Q and R.
4) Find the angle between PQ and PR. This is the angle of the prism (A).
5) Mark M on the side of triangle PQ and also draw a perpendicular to the PQ at M.
6) Place the centre of the protractor at M and along the normal mark an angle of 30° and then draw line up to M.
7) This angle is angle of incidence and note it in a table.
8) Place the prism in its position again.
9) Now fix two pins vertically on the line at point A and B.
10) Look for the images of pins through the prism from the other side and fix another two pins at points C and D in such a way that all the four pins appear to lie along same straight line.
11) Now remove the prism and take out pins.
12) Draw a line joining the two pin-holes formed by the pins to meet surface PR.
13) The angle between the normal at N and the emergent ray is the angle of emergence.
14) Join M and N. A,B,M,N, C, D represent the path of light.
15) Extend both incident, emergent rays tell they meet at a point ‘O’.
16) The angle between these two rays is angle of deviation denoted by ‘D’.
17) Do the same for various angles of incidence such as 40°, 50° etc.
18) If we take angle of incidence along x-axis and the angle of deviation along y-axis we get the graph as shown in figure.
19) The refractive index of prism n = Sin [(A + D)/2] / sin A/2