Question

If work function of a metal is $4.2\,eV$, the cut off wavelength is:

• A. $8000 \,?$
• B. $7000\, ?$
• C. $1472\, ?$
• D. $2950 \,?$

Answer 1

Answer: (D)

$2950 \,?$

Answer 2

Suppose the cut off wavelength is represented by $\lambda_{0}$
So, $\frac{h c}{\lambda_{0}}=$ work function $=W_{0}$
$\frac{h c}{\lambda_{0}}=4.2 \times 1.6 \times 10^{-19}$
$\lambda_{0}=\frac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{4.2 \times 1.6 \times 10^{-19}}$
$=2946 \times 10^{-10} \,m$
$=2950\,?$