Sunday, June 6, 2010


To determine the angle of minimum deviation of a prism using a deviation graph.

Glass prism, drawing board, drawing pins, sheets of paper, cello tape, protractor and a pencil.

On increasing the value of the angle of incidence i, the angle of deviation ẟ first decreases to a minimum value and then increases.


1. Fix a sheet of paper on the drawing board with cello tape.
2. Place the glass prism and draw its boundary ABC.
3. Remove the prism from its place..
4. Mark a point O as shown in the figure and draw the normal NON’ through O.
5. Draw a line XO such that the angle of incidence XON is 40 degrees.
6. Place the glass prism back to its original position.
7. Fix two pins along the line XO.
8. Looking through the other side AC of the prism, fix two more pins such that these two pins and the images of the previous two pins seem to be in a straight line.
9. Remove the pins and the prism and draw the refracted and the emergent rays OY and YZ.
10. Produce the incident ray forward and the emergant ray backward to get the angle of deviation ẟ.
11. Take four more observations with different values of angles of incidence each time.
12. Plot a graph between angle of incidence on the x- axis and the angle of deviation on the y – axis.
13. Determine the angle of minimum deviation from the graph.


1. The prism must be correctly placed on the out line for all the observations.
2. Pins must be vertical.
3. While viewing through the other end of the prism, the feet of the pins must be adjusted to form a straight line and not the heads.

The angle of minimum deviation is found to be ____