A monochromatic point source radiating wavelength `6000Å` with power `2` watt, an aperture A of diameter `0.1 m` and a large screen SC are placed as shown in fig, A photoemissive detector D of surface area `0.5 cm^(2)` is placed at the centre of the screen . The efficiency of the detector for the photoelectron generation per incident photon is `0.9`

(a) Calculate the photon flux at the centre of the screen and the photo current in the detector.
(b) If the concave lens L of focal length `0.6` m is inserted in the aperture as shown . find the new values of photon flux and photocurrent Assume a uniform average transmission of `80%` from the lens .
(c ) If the work function of the photoemissive surface is `1eV` . calculate the values of the stopping potential in the two cases (within and with the lens in the aperture).