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  4. Sodium and copper have work functions 2.3 eV and 4.5 eV respectively. Then the ratio of the wavelengths is nearest to :

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Q. Sodium and copper have work functions $2.3 eV$ and $4.5 eV$ respectively. Then the ratio of the wavelengths is nearest to :

AIEEEAIEEE 2002Dual Nature of Radiation and Matter

Solution:

Work function $W=\frac{h c}{\lambda}$
[Here they are interested in asking threshold wavelength ]
where, $h=$ Planck's constant, $c=$ velocity of light.
Therefore, $\frac{W_{ Nd }}{W_{ Cu }}=\frac{\lambda_{ C.u }}{\lambda_{ Va }}$
$\frac{\lambda_{N a }}{\lambda_{ Cu }}$
$=\frac{W_{ Cu }}{W_{ Na }}=\frac{4.5}{2.3}=2$ (nearly)