(a) Find the intensity at a point on the screen in Young's double slit experiment, at which the interfering waves of intensity $I_0$ each, have a path difference of (i) $\frac{\lambda}{3}$, and (ii) $\frac{\lambda}{2}$. OR (b) A point source of light in air is kept at a distance of $12\text{ cm}$ in front of a convex spherical surface of glass of refractive index $1\cdot5$ and radius of curvature $30\text{ cm}$. Find the nature and position of the image formed.