(a) What are coherent sources ? Why they are necessary for observing stable interference pattern ? Draw a graph showing the variation of intensity of light with the position on the screen in Young's double-slit experiment. (b) Find the intensity of light at a point on the screen when two interfering waves of the same intensity ($I_0$) have a path difference of (i) $\frac{\lambda}{4}$ and (ii) $\frac{\lambda}{3}$. OR (a) Draw a labelled ray diagram of a refracting telescope when it forms image of a distant object at infinity. Derive expression for its magnifying power. (b) (i) In a telescope the objective has much larger aperture than the eye piece. Why ? (ii) Write two advantages of reflecting telescope over refracting telescope.