15-01-2013, 11:01 AM
SPECTROMETER - Refractive Index Of Glass prism
SPECTROMETER.doc (Size: 149.5 KB / Downloads: 53)
AIM:
To determine the angle of the given glass prism and the angle of minimum deviation using a spectrometer and hence to determine the refractive index of the material of the glass prism.
APPARATUS:
Spectrometer, sodium vapour lamp, given glass prism, spirit level and reading lens.
ADJUSTING THE COLLIMATOR FOR PARALLEL RAYS:
The slit of the collimator to catch the image of the slit in the telescope , the distance between the collimating lens and the slit is adjusted by means of the side screw of the collimator till a well defined image of the slit is seen. The vertical cross wire is adjusted for coincidence with the image of the slit without any parallax. Since the telescope has already been adjusted for parallel rays, the light emerging from the collimator will be parallel beam.
TO FIND THE ANGLE OF THE PRISM (A):
The given prism is placed on the prism table, with its refracting edge (i.e.) the triangular edge, facing the collimator, the opaque rectangular base facing the telescope, near the centre of the prism table, bisecting the rays of light coming from the collimator. The two images reflected from the two refracting faces of the prism are first located with the naked eye and prism is fixed. The telescope is brought to one side to receive the first image in the field of view and clamped. The point of intersection of the cross wires is made to coincide with the slit by adjusting the tangential screw of the telescope. Care is taken to see that the vernier clamping screw is tight so that the vernier scale does not move. The readings of the circular main scale and the vernier scale are taken on both the verniers. The total is calculated for each vernier. Let them be R1 for both cases. Keeping the prism table undisturbed, the telescope screw is released and the telescope is rotated to the other side to receive the second reflected image and the readings on the two verniers are noted and the total reading R2 is calculated as before. The differences between the two readings of the reflected rays give 2A. From this, the angle of the prism (A) is calculated.The experiment is repeated by altering the position of the vernier and taking the measurements as before. The readings are tabulated and the mean value of A is found out.
TO FIND THE ANGLE MINIMUM DEVIATION
(D)
The prism is placed at the centre of the prism table with one refracting face almost parallel to the axis of the collimator (the refracting edge towards the telescope) so that the light rays from the collimator are almost grazing the face (i.e. the angle of incidence is nearly 900 . The refracted ray coming out of the other refracting face is viewed- first located with naked eye and then through the telescope. Keeping looking through the telescope, the prism table is slowly rotated such that the refracting edge moves towards the collimator side (from the telescope side). The refracted image moves towards the collimator side (from the telescope side). The refracted image moves towards the direct ray position and the angle of deviation decreases. The telescope is also moved to follow the image. At one stage the image stops, turns back and moves in the opposite direction. The position of the image where it turns back is the minimum deviation position. When the image turns, the telescope is adjusted so that the vertical cross wire coincides with the image at this position. The readings of the verniers are noted as R3 (for both cases).
The prism is removed and the image is viewed directly through the telescope. The telescope is adjusted and fixed at the position when the vertical crosswire coincides with the direct image. The readings of the two verniers are noted as R4(for both the cases). The difference R3~R4 (in both the verniers) gives the angle of minimum deviation D.