3. A plano-concave lens () of diameter D=20.0cm is placed on a flat glass plate as shown in Fig 3.3. Its curved surface, with radius of curvature R=8.00 m, is on the bottom. The lens is illuminated normally from above with parallel light generated by 86Kr (), of which the wavelength is 606nm. Find the order and the radius of the bright circular fringe closest to the center. (The largest bright fringe closest to the edge is the first order. Make use of the approximation: sin 0, cos 0 ~ 1 - 0/2, where is small and in rad. Note half-wave loss!) D Fig. 3.3