Monochromatic radiation of wavelength 640.2 nm (1nm = 10–9 m) from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage.

Given:


Wavelength of light, λ = 640.2 nm = 640.2 × 10-9m


Stopping potential, V0 = 0.54 V


Let frequency of light = v


Work function, Φ0 is given by,


Φ0 = hv-eVs


Φ0 = …(1)


Where,


h = Planck’s constant = 6.6 × 10-34Js


c = speed of light = 3 × 108m


λ = wavelength of light


e = charge on each electron = 1.6 × 10-19C


Φ0 =


Φ0 = 2.229 × 10-19J or 1.39 eV


Wavelength of radiation emitted by iron, λ’ = 427.2 nm


λ’ = 4.27.2 × 10-9m


The new stopping potential can be found using equation (1),


Let new stopping potential = v’


By putting the values in equation (1) we get,


Φ0 =


→


→


→ v’ = 1.5 eV


Therefore, the new stopping potential is 1.5 eV.


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