Physics - Modern Physics Question with Solution | TestHub

According to Einstein's photoelectric equation, the maximum kinetic energy is given by $K_{\max }=h\nu -\Phi$, where $\Phi$ is the work function.

For the initial case:
$h\nu =\Phi +0.4$

When the frequency is increased by $20\%$, the new frequency is ${\nu }^{\prime }=1.2\nu$. The new maximum kinetic energy is $0.7\text{ eV}$.
$h(1.2\nu )=\Phi +0.7$

Substituting $h\nu =\Phi +0.4$ into the second equation:
$1.2(\Phi +0.4)=\Phi +0.7$

$1.2\Phi +0.48=\Phi +0.7$

$1.2\Phi -\Phi =0.7-0.48$

$0.2\Phi =0.22$

$\Phi =\frac{0.22}{0.2}=1.1\text{ eV}$

Answer: 1.1 eV