Question

Work function of a metal is $2.5\text{eV}$ (Lithium). Which of the following radiations can emit photoelectrons, when incident on its surface.
Radiation Wavelength($\lambda$)
$A$ $2000\mathring{\text{A}}$
$B$ $3000\mathring{\text{A}}$
$C$ $5000\mathring{\text{A}}$
$D$ $6000\mathring{\text{A}}$

• A. Only $D$
• B. Only $A,B$ and $C$
• C. Only $A$ and $B$
• D. Only $C$

Answer 1

The correct option is C Only $A$ and $B$

Work function is given as
${\varphi }_0=\frac{hc}{{\lambda }_0}$
Threshold wavelength for Lithium is,
${\lambda }_0=\frac{hc}{{\varphi }_0}=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{2.5\times 1.6\times {10}^{-19}}=4950\mathring{\text{A}}$

We know if $\lambda >{\lambda }_0\to$No photo emission will occur.

$\therefore$ Radiations $C$ and $D$ will not lead to any photoelectric effect

Hence, $\left(C\right)$ is the correct answer.

Why this question? (A typical Question) Concept: Threshold wavelength is the max wavelength for which photoelectric effect is observed above which no photoelectrons will be emitted.