why spectrum due to diffraction appears in reverse order as compared to specrtum obtained by dispersion?

Answer 1

The mechanisms for spectral spread are completely different, so there's no reason for thinking they should be the same direction in the first place.

A diffraction grating deflects a given wavelength in the direction that results in maximum constructive interference of the individual wavelets diffracted in all directions from each ruling. It's a purely geometrical effect requiring only the Pythagorean theorem to calculate the angle; the material that the grating is made of has no effect on the angle. Start drawing triangles and you'll see that max interference occurs with blue first as the angle is increased, so blue deflects least.

Refraction however, *is* a molecular effect due to harmonic excitation of the bound electrons. The optical resonance of most solids occurs in the ultraviolet (that's when the solid absorbs photons strongly and electrons get ejected). So, just like with classical harmonic oscillation, as the (radiation) driver frequency approaches resonance (changes from red to blue), oscillations increase in amplitude and the refractive index, therefore, increases. Blue, therefore, deflects *most*.

Answer 2

Ram, the degree to which a wave diffracts depends on its wavelength. When the grating dimensions are of the order of a micron, red wavelength is larger and closer to slit dimensions than blue and hence diffracts to a larger extent.
But dispersion is stronger for blue (dn/d(lambda) < 0) and hence bends stronger than red when it encounters the same glass prism.