Ultraviolet light of wavelength `2271Å` from a 100W mercury source irradiates a photocell made of molybdenum metal. If the stopping potential is 1.3V, estimate the work function of the metal. How would the photocell respond to a high intensity `(~10^(5)Wm^(-2))` red light of wave length `6328Å` produced by a He-Ne laser?

Explanation

Given , for UV light , `lambda = 2271 Å = 2271 xx 10^(-10) ` m
` V_(0) = 1.3 V, P = 100 W , h = 6.63 xx 10^(-34) J - s, c = 3 xx 10^(8) ` m/s
From Einstein's equation ` E = KE + phi _(0) , hv = eV_(0) +phi_(0)`
` phi _(0) = (hc)/lambda - eV_(0) = (6.63xx10^(-34)xx3xx10^(8))/(2271 xx10^(-10))- 1.6 xx10^(-19) xx1.3 = 8.758 xx10^(-19) - 2.08xx10^(-19)`
` phi _(0) = (6.678 xx 10^(-19))/(1.6 xx 10^(-19) ) eV = 4.17 eV :. phi_(0) = 4.2 eV`
Given , for red light , ` lambda = 6328 Å= = 6328 xx10^(10)` m
` E = (hc)/lambda = (6.63 xx 10^(-34) xx 3 xx 10^(8))/(6328 xx 10^(-10)) = (3.143 xx 10^(-19))/(1.6 xx 10^(-19)) eV:. E = 1.96 eV`
Here E ` lt phi_(0)` So , the photocell willnot respond to this red light . (It is independent of intensity )