Question

The work function of a substance is $4.0 \,eV$. The longest wavelength of light that can cause photoelectron emission from this substance is approximately

• A. 540 nm
• B. 400 nm
• C. 310 nm
• D. 220 nm

Answer 1

Answer: (C)

The minimum energy of incident radiations, required to eject the electrons from metallic surface is defined as work function of that surface.
$W_{0}=h v_{0}=\frac{h c}{\lambda_{0}}$
where $\lambda_{0}=$ threshold wavelength.
Work function in electron volt $W_{0}( eV )$
$ W_{0} =\frac{12400}{\lambda_{0}(\mathring{A})} eV $
$ \therefore 4.0 =\frac{12400}{\lambda_{0}} $
$ \Rightarrow \lambda_{0} =310 \,nm$