A 100 W light source emits uniformly in all directions. A photodetector having a circular active area whose diameter is 2 cm is placed 1 m away from the source

Q. A 100 W light source emits uniformly in all directions. A photodetector having a circular active area whose diameter is 2 cm is placed 1 m away from the source, normal to the incident light. If the responsivity of the photodetector is 0.4 A/W, the photo-current generated in the detector, in units of mA, is

             (A) 1               (B) 4                           (C) 100                         (D) 400

Ans: 1

Sol:

The whole power of 100 W will not fall on the active area

Intensity of light at a distance ‘d’ from an isotropic source is given by