2. Light waves of the wavelength of 650 nm and 500 nm produce interference fringes on a screen at a distance of 1m from a double-slit of separation 0.5 mm. Find the least distance of a point from the central maximum where the bright fringe occurs due to both sources coincide? 3. Suppose (100), (110) and (111) are three simple cubic lattice. Find the ratio of the lattice the separation between the successive planes of these crystals.