A mercury arc lamp provides 0.10 W of UV radiation at a wavelength of `lambda = 2537Å`(all other wavelenghts having been absorbed by filters). The cathode of photoelectric device (a photo-tube) consists of potassium and has an effective area of `4cm^(2)`. The anode is located at a distance of 1 m from radiation source. The work function for potassium is `phi_(0) = 2.22eV`.
(a) According to classical theory, the radiation from the arc spreads out uniformly in space as spherical wave. What time of exposure to the radiation should be required for a potassium atoms(radius 2 Å) in the anode to accumulate sufficient energy to eject a photo-electron?
(b) What is the energy of a single photon from the source?
(C) What is the flux of photons(number per second) at the cathode? To what saturation current does this flux correspond if the photo-conversion efficiency is 5% (i.e. if each photon has a probability of 0.05 of ejecting an electron).
(d) what is the cutt off potential `V_(0)`?

Answer

B

Explanation

Cut off potential `= ((4.897 - 2.22)eV)/(e) = 2.69` volts.