11

EXPT NO: 11. PRISM

Aim :         To determine angle of minimum deviation for a given prism by plotting a graph between angle of incidence and angle of deviation.

Apparatus : Prism, pins, sheet of paper, drawing board, protractor, scale, drawing pins etc.

Diagram :

Procedure :

      Take a sheet of white paper and fix it on the drawing board by using drawing pins.

      Place the prism on the paper and draw its out line ABC as shown in figure.

      Draw the normal DE to face AB at D. Draw another line DF making an angle of 35° with normal at D. This is the incident ray. I

      Fix two pins P and Q at a sufficient distance apart on line DF as shown in figure.

      Place the prism exactly as before on its outlines ABC.

      Now observe from face AC and fix to pins R and S in such a way that the refracted images of pins P and Q and pins R and S are in the same straight line.

      Remove the prism and pins, join PQ and produce forward as DX. Similarly join RS and produce backward as ZY. They intersect at point Y. Then, angle of deviation. Measure the angle of deviation.

      Draw the normal to face AC at the point of emergence Z as .

      Repeat the experiment by changing the angle of incidence as 40°, 45°, 50°, 55°, 60°. Plot a graph of  against  .

      From the graph, find the minimum value of angle of deviation, which is angle of minimum deviation .

Observation :

Angle of prism = A = 60°

Observation table :

Obs. No.	Angle of incidence	Angle of deviation
01	350
02	400
03	450
04	500
05	550
06	600

Graph :

Result :

1)     Angle of minimum deviation =  =…………………..

Precautions :

1)     The distance between pair of pins should be about 4 cm on each side of prism.

2)     All pins should be exactly vertical.

3)     To adjust the pin in a straight line, observe the base of the pins and not the heads.

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