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Q.
The work function of a metal $2.8\, eV$. Its threshold wavelength will be :
J & K CETJ & K CET 2002
Solution:
The work function $(W)$ is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface.
It is given by $W=\frac{h c}{\lambda}$ where $h$ is Plancks constant, $c$ the speed of light and $\lambda$ the threshold wavelength.
Given, $W=2.8\, eV =2.8 \times 1.6 \times 10^{-19} J$
$h=6.6 \times 10^{-34} J - s,\, c=3 \times 10^{8} m / s$
$\lambda=\frac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{2.8 \times 1.6 \times 10^{-19}}$
$\lambda=4433 \times 10^{-10} m$
$\Rightarrow \lambda=4433\,\mathring{A}$