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A cobalt (Co) plate is placed at a distance of 1 m from a point source of power 1 W. Assume a circular area of the plate of radius, r= 1 A is exposed to the radiation and ejects photo electrons. The light energy is considered to be spread uniformly and the work function of cobalt is 5 eV. The minimum time the target should be exposed to the light source to eject a photoelectron (Assuming no reflection losses) is |
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Answer» 320 s Source power , P = I W Intensity of light source at a distance of 1m is given by `I = (P)/(4pir^2) = (1)/(4pil^2) = (1)/(4pi) W//m^2` Power absorbed by the circular plates of RADIUS 1Å is , ` P = I xx ` area of PLATE ` = (1)/(4pi) xx pi r^2 ""[ becauser = 1 Å = 10^(-10) m ]` ` P = 1/4 (10^(-10) )^2 (10^(-20) )/(4)` Work function of cobalt `W = 5 eV = 5 xx 1.6 xx 10^(-91) J` `P = W/t` `t = W/P` ` THEREFORE t = (5 xx 1.6 xx 10^(-19) )/( (10^(-20 ) )/(4) )= 320 s ` |
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