Question

Finding the refractive index of a prism

Answer 1

ANSWER

AIM: To determine refractive index of the material of the given prism by the method of i-d curve.

Materials required: Prism, piece of white chart of size 20 x 20 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 using a pencil. Remove the prism.

It is a triangle. Name its vertices P, Q and R.

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 the PQ at M.

Place the centre of the protractor at M and along the normal mark an angle of 30° and then draw line up to M.

This angle is angle of incidence and note it in a table.

Place the prism in its position again.

Now fix two pins vertically on the line at point A and B.

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.

Now remove the prism and take out pins.

Draw a line joining the two pin-holes formed by the pins to meet surface PR.

The angle between the normal at N and the emergent ray is the angle of emergence.

Join M and N. A,B,M,N, C, D represent the path of light.

Extend both incident, emergent rays tell they meet at a point ‘O’.

The angle between these two rays is angle of deviation denoted by ‘D’.

Do the same for various angles of incidence such as 40°, 50° etc.

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.

The refractive index of prism μ=sin2Asin[2(A+D)]

RESULT: Refractive index of the material of the prism