Finding the angle of prism. 1) Place the prism on the turn table in such a way that the refracting edge A coincide with the centre of the table and one of the refracting or polished face say AB remains perpendicular to the line joining the screws Q and R. 2)Now , rotate the prism table to bring the refracting edge A of the prism towards the collimator so that the light from the collimator falls equally on both faces AB and AC bounding the refracting angle A. the prism angle is then clamped. 3) determine the least count of the spectrometer. 4) rotate the telescope to the right side to receive the light reflected from the face AB and focus the cross wire on the image of the slit . Now by tangent screw , the telescope is moved slowly until its vertical cross wire coincide with the image . The readings of the two verniers of the spectrometer are taken. 5) Now rotate the telescope to the left side to receive the light reflected from the face AC and again , focus the cross wire on the image of the slit by means of the tangent screw . The readings of the two verniers are again noted. 6) the difference of the two vernier readings of the same vernier for two positions of the telescope gives twice the refracting angle A of the prism . Thus , half of this angle will give the value of the angle A of the prism.
Angle of minimum deviation:- The prism is placed on the turned table G such that the axis of rotation of the turn table passes through the centre of the prism one of its polished face AB is turned towards the collimator in such a way that light after refraction emerges out from the other polished face AC and the dispersed light is received in the telescope .Now the turn table is slowly rotated in such a direction that the deviation of the rays decreases . The telescope is also rotated so as to keep the spectrum in the field of view . The rotation is continued until the spectrum comes to momentary rest and then turns back. the position of the prism at which the spectrum is just at rest is the position of the minimum deviation . The turn table is clamped in this position. the position of the telescope is so adjusted by means of the tangent screw that the intersection of the cross wires is exactly at the centre of the spectral line under investigation. the position of the telescope is read on both the verniers. Now the prism is removed and the telescope is turned to receive the direct image of the slit on the cross wires and its position is again read. The difference between the readings in the two positions of the telescope gives the angle of minimum deviation.