Monochromatic radiation of wave length 640.2 nm `(1nm=10^(-9)m)` from a neon lamp irradiates a photosenstive material made of calcium or tungsten. The stopping voltage is measured to be 0.54V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photocell. Predict the new stopping voltage.

Explanation

Here, For neon lamp, `lambda=640.2nm =640.2xx10^(-9)m, V_(0)=0.54V. We know, eV_(0)=(hc)/lambda-phi_(0)`
`:.` Work function, `phi_(0)=(hc)/lambda-eV_(0)=(6.63xx10^(-34)xx3xx10^(8))/(640.2xx10^(-9))-1.6xx10^(-19)xx0.54`
`=3.1xx10^(-19)-0.864xx10^(-19)J=2.236xx10^(-19)J= (2.236xx10^(-19))/(1.6xx10^(-19))eV=1.4eV`
For iron lamp, `lambda=427.2nm =427.2xx10^(-9)m`,
`:. eV_(0)=(hc)/lambda-phi_(0)`
`=(6.63xx10^(-34)xx3xx10^(8))/(427.2xx10^(-9))-2.236xx10^(-19)`
`=4.649xx10^(-19)-2.236xx10^(-19)J=2.413xx10^(-19)J`
`:.` Stopping potential,
`V_(0)=(2.413xx10^(-19))/e= (2.413xx10^(-19))/(1.6xx10^(-19))eV=1.55V`