= 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 $eV_{\rm stop}=h\nu-\phi$ 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 $\phi$ the <work function>. The retarding-potential measurement distinguishes the <electron> <energy> from the photocurrent; the two respond differently to changes of the light.
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