Question

The work function of a metal is 4.2eV, its threshold wavelength will be

• A. 4000$\stackrel{0}{A}$
• B. 3500$\stackrel{0}{A}$
• C. 2960$\stackrel{0}{A}$
• D. 2500$\stackrel{0}{A}$

Answer 1

Answer: (C)

2960$\stackrel{0}{A}$

Given:

$e=1.6\times {10}^{-19}C$

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

$\varphi =4.2eV=4.2\times 1.6\times {10}^{-19}=6.72\times {10}^{-19}J$

$h=6.63\times {10}^{-34}J.s$

Threshold wavelength $={\lambda }_o=\frac{hc}{\varphi }=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{6.72\times {10}^{-19}}=2.960\times {10}^{-7}m=2960A^o$