(a) A Plano-convex lens is made of glass (n=1.5) with one surface flat and the other having a radius of 20 centimeter. Determine what is the focal length of the lens?
(b) A sodium surface is bombarded by photons that have a wavelength of 200 nm. Compute the energy of the photons. (h=6.62x10-34J.s, 1eV = 1.602x10-19 J, c =3x108)