Dear Rajan,

here f₀ is threshold frequency

so,         1.2 = hf - ψ

 and,     4.2 = 1.5hf - ψ

solve for ψ

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Askiitians Expert

Anil Pannikar

IIT Bombay

when frequency is f then max kinetic energy is 1.2ev

 (KE) = hf  -@                       (@ is work function)

 1.2  = hf - @ .......1

when frequency increased by 50% then new frequency is f+f/2 =3f/2 and

(KE)max = 3hf/2 - @

4.2 = 3hf/2 - @ ................2

from 1 & 2

@ = 4.8ev ans