incident on both reflecting faces of the prism and is reflected. • The telescope is rotated towards left to obtain reflected image of the slit from face of the prism and is fixed. • Using tangential screws, the telescope is adjusted until the vertical cross-wire coincides with the reflected image of the slit. • The main scale reading and vernier coincidence are noted from both vernier scales.
• The telescope is now rotated towards right to obtain the reflected image from face of the prism. As before, the readings are taken. • The difference between the two readings gives 2A from which the angle of the prism A is calculated. ) Determination of angle of minimum deviation (D) • The prism table is rotated such that the light emerging from the collimator is incident on one of the refracting faces of the prism, gets refracted and emerges out from the other refracting face.
• The telescope is turned to view the refracted image. • Looking through the telescope, the prism table is rotated in such a direction that the image moves towards the direct ray. XII- - ) XII- - ) - - - - PRACTICAL • At one particular position, the refracted ray begins to retrace its path. The position where the refracted image returns is the position of minimum deviation.
• The telescope is fixed in this position and is adjusted until the vertical cross-wire coincides with the refracted image of the slit. • The readings are taken from both vernier scales. • The prism is now removed and the telescope is rotated to obtain the direct ray image and the readings are taken. • The readings are tabulated and the difference between these two readings gives the angle of minimum deviation D.
• From the values of A and D, the refractive index of the material of the glass prism is deter- mined. Least count MSD = ′ Number of vernier scale divisions = For spectrometer,