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 $=3\times {10}^{8}m/s$).

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

Answer 1

The correct option is B $650nm$

The minimum wavelength to cause photoelectric effect $=\frac{hc}{E_{max}}$

$=\frac{4\times {10}^{-15}\times 3\times {10}^8}{5}m$

$=240nm$

The maximum wavelength to cause photoelectric effect $=\frac{hc}{E_{min}}$

$=\frac{4\times {10}^{-15}\times 3\times {10}^8}{2}m$

$=600nm$

Hence light of wavelength $650nm$ is not acceptable.