A photelectric meterial having work-function ${\varphi }_0$ is illuminated with light of wavelegth $\lambda \left(\lambda <\frac{hc}{4_0}\right).$ The fastes photoelectron has a de Broglie wevelength ${\lambda }_d.$ A change in wavelength of the incident light by $\Delta \lambda$ results in a change $\Delta {\lambda }_d$ in ${\lambda }_d.$ Then the ration $\Delta {\lambda }_d/\Delta \lambda$ is proportional to

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