(a) Draw a labelled ray diagram showing the formation of an image by an astronomical refracting telescope in normal adjustment. Hence, obtain the expression for its magnifying power. OR (b) A plane wavefront of light of wavelength '$\lambda$' is incident normally on a narrow slit of width 'a' and a diffraction pattern is observed on a screen at a distance 'D' from the slit. (i) Depict the intensity distribution in the pattern observed. (ii) Obtain the expression for the first maximum from the central maximum.