Question

The work function of metals is in the range of $2eV$ to $5eV$. 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 $=3\times 10^{8}m/s$)

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

Answer 1

Answer: (B)

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