"A 20 W bulb emits light of wavelength 5000 Å shining on a metal surface at a distance of 2 m. Assuming the metal surface has a work function of 2 eV, and each atom on the surface can be treated as a circular disk with a radius of 1.5 Å. What is the time required for the atom to receive energy equal to the work function of 2 eV?"