The minimum wavelength of photon is $5000 \mathrm{\AA },$ its energy will be 

A photosensitive metallic surface has work function $h{\nu }_0$. If photons of energy $2h{\nu }_0$ falls on this surface, the electron come out with a maximum velocity of $4\times {10}^6 \mathrm{m} {\mathrm{s}}^{-1}$.When the photon energy is increased to $5h{\nu }_0$, then maximum velocity of photoelectrons will be: 

A $5 \mathrm{W}$ source emits monochromatic light of wavelength $5000 \stackrel{\circ }{\mathrm{A}}$. When placed $0.5 \mathrm{m}$ away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of $1.0 \mathrm{m}$, the number of photoelectrons liberated will be reduced by a factor of