Page 81 - Physics - XII

1. With the help of drawing pins, fi x a white sheet of paper on the drawing board.
2. By using a sharp pencil, draw a straight line XX' nearly at the centre of the paper and parallel to the

length of the paper.
3. On the line XX' mark points O , O , O .......... at the suitable distances (about 5 or 6 cm) and draw
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normals N O , N O , N O .......... on points O , O , O , .......... as shown in Fig. 5.2.
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4. Draw straight lines T O , T O , T O .......... making angles of 30°, 35°, 40°, .......... 60° with the normals
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N O , N O , N O .......... respectively using a protractor.
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5. Place the prism with one of its refracting surfaces on the line XX' and Point O in the middle of the
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prism. Mark the boundary of the prism ABC as shown in Fig. 5.2.
6. Fix two pins P and Q vertically on the line T O (incident ray line). The distance between the pins
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should be about 10 to 12 cm. Now, view the images of P and Q through the face AC of the prism.
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Keeping one eye closed and bring open eye in the line of the two images of P and Q . Fix two pins R 1
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and S on the paper vertically and about 12 cm apart such that the tips of these pins and the tips of the
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images of the incident ray pins P and Q , all lie on the same straight line.
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7. Remove the pins P , Q and R , S and encircle their pin pricks on the paper sheet with the help of a
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pencil.
8. Join the pin pricks R and S and produce it backwards to meet the incident ray T O produced (as
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shown by dotted line in Fig. 5.2). R S is the emergent ray corresponding to the incident ray T O . Mark
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arrow heads to show the direction of incident and emergent rays.
9. Measure the angle E F S with the help of the protractor. This gives the angle of deviation D .
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10. Repeat the steps 5 to 9 for diff erent values of angle of incidence and measure the corresponding angle
of deviation. Take the values of angle of incidence ranging from 30°, 35°, 40°, 45°, 50°, 55°, and 60°.
11. Measure angle BAC in the boundary of the prism. This gives angle of the prism A.
12. Record all the observations in the observation table.
Observations
1. Angle of prism ‘A’ = _____
2. Least count of the protractor = _____ degree
Table for angle of incidence and angle of deviation
S. No. Angle of incidence, i (degrees) Angle of deviation, D (degrees)
1. 30°
2. 35°

3. 40°
: :
: :

7. 60°

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