Question

The work function of metals is in the range of $2\, eV$ to $5\,eV$. Find which of the following wavelength of light cannot be used for photoelectric effect. (Consider, Planck constant $= 4\times 10^{-15}eVs$, velocity of light $= 3\times 10^8m/s$)

• A. 510 nm
• B. 650 nm
• C. 400 nm
• D. 570 nm

Answer 1

Answer: (B)

Given that, $2\, eV \leq \phi \leq 5\, eV$
Wavelength corresponding to minimum and maximum values of work function are
$\lambda_{\max } =\frac{h c}{E}=\frac{4 \times 10^{-15} \times 3 \times 10^{8}}{2}$
$=6 \times 10^{-7} m =600\, nm$
and $\lambda_{\min } =\frac{h c}{E}$
$=\frac{4 \times 10^{-15} \times 3 \times 10^{8}}{5}$
$=2.4 \times 10^{-7} m =240\, nm$
Clearly, wavelength range would be
$240 \leq \lambda \leq 600$
Thus, wavelength $650\, nm$ is not suitable for photoelectric effect.