 Solution

In a photoelectric effect experiment, illuminate a clean metal photocathode with monochromatic light and use a retarding voltage to measure the maximum emitted-electron kinetic energy. The relation $e{V}_{\mathrm{stop}}=h\nu -\varphi$ has a frequency threshold and a slope $h$. Increasing intensity chiefly changes the number emitted, while increasing frequency increases the available energy per emission. This supports energy transfer in units $h\nu$, with $\varphi$ the work function. The retarding-potential measurement distinguishes the electron energy from the photocurrent; the two respond differently to changes of the light.