A photoelectric material having work-function $\phi_0$ is illuminated with light of wavelength $\lambda\left(\lambda<\frac{\mathrm{hc}}{\phi_0}\right)$. The fastest photoelectron has a de Broglie wavelength $\lambda_{\mathrm{d}}$. A change in wavelength of the incident light by $\Delta \lambda$ results in a change $\Delta \lambda_{\mathrm{d}}$ in $\lambda_{\mathrm{d}}$. Then the ratio $\Delta \lambda_{\mathrm{d}} / \Delta \lambda$ is proportional to