reads - and is fixed. This is the reading for the direct ray. • The telescope is then rotated (anti-clockwise) through an angle of and fixed (Figure (a) ). • Now the plane transmission grating is mounted on the grating table.
• The grating table alone is rotated so that the light reflected from the grating coincides with vertical cross-wire of the telescope. The reflected image is white in colour (Figure (a) ). • Now the vernier disc is released. The vernier disc along with grating table is rotated through an angle of in the appropriate direction such that the light from the collimator is incident normally on the grating (Figure (a) ).
) Determination of wave length of the constituent colours of the mercury spectrum • The telescope is released and is brought in line with the collimator to receive central direct image. This undispersed image is white in colour. • The diffracted images of the slit are observed on either side of the direct image. • The diffracted image consists of the prominent colours of mercury spectrum in increasing order of wavelength.
• The telescope is turned to any one side (say left) of direct image to observe first order dif- fracted image. • The vertical cross-wire is made to coincide with the prominent spectral lines (violet, blue, yellow and red) and the readings of both vernier scales for each case are noted. • Now the telescope is rotated to the right side of the direct image and the first order image is observed. • The vertical cross-wire is made to coincide with the same prominent spectral lines and the readings of both vernier scales for each case are again noted.
• The readings are tabulated. • The difference between these two readings gives the value of 2θ for the particular spectral line. • The number of lines per metre length of the given grating N is noted from the grating. • From the values of N, n and θ, the