LAB MANUAL PHYSICS-XIl ‘Angle of deviation Fig. 10.3 (a) A ray of light passing through a prism. (b) Plot of variation of 8 versus i. decreases and reaches a minimum value ,, and then increases. Clearly, any given value of 6 corresponds to two angles of incidence i and ‘The minimum value of the angle of deviation suffered by @ ray on passing through a prism is called the angle of minimum deviation and is denoted by 8, or D,,. The advantage of placing the prism in the position of minimum deviation is that the image is brightest in this position. [El Relation between Refractive Index and Angle of Minimum Deviation When a prism is in the position of minimum deviation, a ray of light passes symmetrically (parallel to the base) through the prism so that i 5-6, rer, At+bzite EXPERIMENT an To determine the angle of deviation of the lass pris Ss ¢ angle of incidence and angle of deviation (and hence to fe eRe beet _ the material of the prism). _APPARATUS AND MATERIAL REQUIRED Drawing board, triangular glass prism, alpins, paper sheets and a graph paper. [L tHeoryworkinc Formutac A+b, =i4i or From Snell's law, the refractive index of the material of the prism will be or By knowing the values of A and 6,,,, the refractive index 11 of the glass prism can be determined. =] to find the refractive index of drawing pins/cellotape, protractor, a rules, i 1. When a ray of light passes through a prism, Angle of prism + Angle of deviation = Any 1422 Chopler 10 : REFRACTION THROUGH A GLASS PRISM Asi increas, ie ae decreases, becomes minimum and then increases Inthe position oF minimum deviation 8, , the ray of light passes symmetrically, ie., parallel to the base 5° that jeeand r=?’ ou ‘The relation between pt and 6,, is l | DAGRAMS Angle of deviation ime © Fig. 104 (a) Refraction of light through a prism (0) Variation of A with 6 fora glass prism. [ procepure i 2 . Mark seven points 0, , 03, « Using a protractor, draw straight lines P,Q,» P:Or Fix a sheet of white paper on a drawing board with drawing pins/cellotape. Using a sharp pencil, draw a long straight line XY in the middle and parallel to the length of the paper. , at equal distances of 6 em on the line XY. ,, N,O, on the line XY. Draw normals N,O,, NO, P,O, corresponding to the incident rays making angles of incidence at 30°, 35°, 40", 50%, 55° and 60° respectively with the normals. By keeping a ruler along a line ABlies on the line XY with poit 7 boundary of the prism with a sharp pencil x Ns ™ : XY, place the prism along the ruler so that its refracting face int O, in the middle of AR as shown in Fig, 1055, Draw the NLAB MANUAL PHYSICS-XIl incident ray line P, 7. Fix two alpins P, and P, vertically, more than 6 cm apart, on the inci y line PLO, sug that pin P, is close to point O,. 8. Close the left eye. Looking into the prism from the opposite refracting face AC, position the right eye in line with images of the feet of pins P, and P,, Now fix pins P, and P, in line with P, and P, as viewed through the prism. Eye should be kept at some distance from the pins so that all the pins can be seen in clear focus simultaneously, as Ee shown in 10.6. Distances P,P, and P,P, should not be less than 6 cm so as to locate the Fig. 10.6 How to trace the path of directions of the incident ray and the emergent a ray through a prism, ray with an accuracy of the order of 1°. 9. Remove the alpins and encircle their pin pricks with a sharp pencil. Draw the line joining , and Py, Produce lines P,P, (forward) and P,P, (backward) so that they meet point M Draw arrowheads on P,P, and P,P, to indicate the directions of incident and emergent rays respectively, 10. Using a protractor, measure the angle of incidence i= 2P,0,N, and also the angle of deviation 8= ZP,MS. 1. By placing the prism at locations O,, O,, O4, Os, O, and O,, repeat the experiments for angles of incidence 35°, 40°, 45°, 502, | 35°, 55° and 60°, respectively. Measure the corresponding angle of deviation in each case and indicate the values of these angles in the diagram. 12. Measure the angle of prism A between the refractin 13. Record all observations in a tabular form, [Losseevarions Angle of prism, A=60° Table 10.1 : 1 faces ABand AC. Different values of i and 8 for a prism Angle of incidence, i | An ¥ ile of deviati ‘| ei al leviation, § 2 = 35° 40° 45° NO ee ery e Fig. 107, we need to 2°and 3°. JAAChapter ‘opter 10 ; REFRACTION THROUGH A GLASS PRISM [cure AND CALCULATIONS Choosing suitable scales, plot a between i and & Take angle of ineidenee along X-axis and angle of deviation § along Y-axis. Draw free hand smooth curve passing through the maximum number of plotted points, Toh 8 graph fordpren ] Draw a tangent on the lowest point of the curve parallel to the X-axis. Read the angle of minimum deviation 6, on the Y-axis of the graph. ‘Angi of deviation | Angle of minimum deviation, 8, = 36 aa 6 Y-axis: 1 dive 02° “io 30, 3a] 40) 0) 0 = Anaiet ens oe)» Fig. 10.7 Variation of angle of deviation with angle | Resuur of incidence for a prism, 1, As the angle of incidence increases, the angle of deviation first decreases, reaches a minimum value and then again increases, as is obvious from the i versus 6 graph. 2. Angle of minimum deviation, 8,, 3. Refractive index of the prism material, 1 Lorecaurs IONS 1. An ink mark should be put on the prism to distinguish the refracting angle A from the other angles and the same angle of the prism should be used throughout. 2. The angle of incidence should not be Jess than 30°, as the ray may get totaly reflected inside the prism. 3. For prism of side 2.5 or 3m, Id not be less 4. Alpins should be fixed vertically and the pin pricks should be encircled immediately after they are removed. While fixing alpins, the feet of pins (and traight line. | > 6 i ines the incident and emergent rays accurately, the pin separations P,P, and P,P, should be more than 6 cm. 7 icate the ¢ incident, * indicate the path ot Ded ud be drawn through the plotted points, the distance between any two pins should not be less than 6 cm, not their heads) must be adjusted in the same the refracted and the emergent rays by arrowheads. 8. A smooth best fitting curve ht Loources OF ERROR 1. Alpins may not be vertical: 2 The feet of the alpins may not be 3. The distances between the pins may of the prism might have dist les may not be accurate. in a straight line. be small : rbed during a given set of observations. 5. Measurement of angl ae , ld 145