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Question
Monochromatic fight of wavelength 198 nm is incident on the surface of a metallic cathode whose work function is 2.5 eV How much potential difference must be applied between the cathode and the anode of a photocell to just stop the photocurrent from flowing?
Solution
λ = 198 nm = 198 × 10-9 m
W = 2.5 eV
∴ Energy of the incident photon
E = `"hc"/lambda = (6.6 xx 10^-34 xx 3 xx 10^8)/(198 xx 10^-9 xx 1.6 xx 10^-19) "eV"`
`= (6.6 xx3)/(198 xx 1.6) xx 100 = 6.25 "eV"`
∴ Energy of the ejected electron = 6.25 - 2.50 = 3.75 eV
∴ The retarding potential to stop the elctron = 3.75 V
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