A beam of electrons of energy E scatters from a target having atomic spacing of 1 $$\mathop A\limits^o $$. The first maximum intensity occurs at $$\theta $$ = 60^o. Then E (in eV) is ______.
(Planck constant h = 6.64 × 10^–34 Js,
1 eV = 1.6 × 10^–19 J, electron
mass m = 9.1 × 10^–31 kg)

When radiation of wavelength $$\lambda $$ is used to illuminate a metallic surface, the stopping potential is V. When the same surface is illuminated with radiation of wavelength 3$$\lambda $$, the stopping potential is $${V \over 4}$$. If the threshold wavelength for the metallic surface is n$$\lambda $$ then value of n will be __________.

A beam of electromagnetic radiation of intensity 6.4 × 10^–5 W/cm² is comprised of wavelength, $$\lambda $$ = 310 nm. It falls normally on a metal (work function $$\phi $$ = 2eV) of surface area of 1 cm². If one in 10³ photons ejects an elctron, total number of electrons ejected in 1 s is 10^x. (hc = 1240 eVnm, 1eV = 1.6 × 10^–19 J), then x is _____.