How do you find experimentally the refractive index of material of a prism.

Objective : To find the refractive index of material of a prism

Materials required : prism, a white sheet, thumb pins, protractor, pencil, scale

Formula used : n =

where n = refractive index of material of prism

A = angle of prism

D = angle of deviation

Procedure

● Place the white sheet of paper on the table. Put the prism on the table such that the triangular base is on the white sheet.

● Trace the triangular base on sheet with the help of pencil.

● Now remove the prism. Name the triangle as P,Q & R.

● With the help of protractor, find the angle between PQ & PR. It is the angle of prism (A).

● On the side PQ, draw a perpendicular at point M.

● Draw the incident ray at an angle say 30 degrees with respect to the normal and extend the line till point M. This angle is angle of incidence.

● Place the prism in the same position as earlier on the trace of the triangular base.

● Fix the pins at point A & B on the incident ray.

● See the images of the pins A & B through the prism and fix pins at point C & D so that all the pins appear to be in same line.

● Remove the prism and draw a line through points C & D. This line is the emergent line and the angle between emergent line & the normal is called angle of emergence.

● Join the points M & N so that the path of light can be traced.

● Now extend the incident ray & emergent ray till they meet at some point O. The angle between the two rays is called angle of deviation (D).

● Repeat the above steps for various angles of incidence and note down the observations.

● The refractive index can be calculated using the formula:

● Plot the graph between angle of incidence & angle of deviation.

Observation Table

Angle of prism (A) = 60 degrees