Question

The work function of metal is in the range of $2eV$ to $5eV.$ Find which of the following wavelength of light cannot be used for photoelectric effect:
(Consider, Plank constant $=4\times {10}^{-15}eVs,$ velocity of light $=3\times {10}^{7}m/s$)

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

Answer 1

The correct option is B $650nm$

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}^3}{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.