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  4. The threshold wavelength of lithium is 8000 A°. When light of wavelength 9000 A° is made to be incident oh it; then the photo electrons

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Q. The threshold wavelength of lithium is 8000 $A^{\circ}$. When light of wavelength 9000 $A^{\circ}$ is made to be incident oh it; then the photo electrons

Dual Nature of Radiation and Matter

Solution:

The wavelength of light is inversely proportional to its frequency, hence. threshold frequency of lithium light is
$v_{0} \propto \frac{1}{8000}$
and the frequency of incident light is
$v \propto 19000$
From above equations
$\frac{1}{8000} > \frac{1}{9000}$
$\Rightarrow v_{0} > v$
Since, threshold frequency is the minimum frequency due to which electron is able to knock out of the surface of metal.
The incident frequency should be more than threshold frequency, therefore no photoelectrons will be emitted.