A low intensity ultraviolet light of wavelength 2250 Å irradiates a photocell made of molybdenum metal. If the stopping potential is $1.5 \mathrm{V}$, calculate the work function of the metal. Will the photocell work if it is irradiated by a high intensity light of wavelength 6875 Å?

The energy (in eV) of the incident photons is

$E=\frac{12375}{2250} \mathrm{eV}=5.5 \mathrm{eV}$

Since, the stopping potential for ejected electrons is $1.5 \mathrm{V}$, so the maximum kinetic energy of ejected photoelectrons is

$K_{\max }=e{V}_s=1.5 \mathrm{eV}$

Applying Einstein's photo electric equation, work function

$\Rightarrow {\varphi }_0=E-K_{\max }=(5.5-1.5) \mathrm{eV}=4 \mathrm{eV}$

Energy (in eV) ) for photons of high intensity light of wavelength $6875 Å$ is

$E\text{'}=\frac{12375}{6875} \mathrm{eV}=1.8 \mathrm{eV}$

Since, $E\text{'}<{\varphi }_0$ , so the photocell will not work even if irradiated by this high intensity light.