A meacury arec lamp provides `0.1` watt of ultra-violet radiation at a wavelength of `lambda=2537 Å` only. The photo tube (cathode of photo electric device) consists of potessium and has an effective area of `4 cm^(2)`. The cathode is located at a distance of `1 m` from the radiation source. The work funcation for potassium is `phi_(0) = 2.22 eV`.
According to classical theory, the radiation from arc lamp spreads out uniformaly in space as spherical wave. What time of expource to the radiation should be required for a potassium atom (radius `2Å`) ub tge cathode to accumulate sufficient enerfy to eject a photo-electron?

Answer

B

Explanation

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