Dual Nature of Radiation and Matter

$\varphi$: work function (minimum energy to free an electron). $KE_{\max}$: maximum kinetic energy of the emitted photoelectrons.

1. Below threshold (f < f₀): zero photoelectrons emitted, no matter how high the intensity is pushed. Each photon carries too little energy, and electrons can’t pool energy from several photons.
2. Above threshold (f ≥ f₀): raising intensity (frequency fixed) increases the number of photoelectrons per second, i.e. the photocurrent, but leaves $KE_{\max}$ (and hence $V_0$) unchanged. Raising frequency instead increases $KE_{\max}$, but doesn’t by itself change the current if intensity is unchanged.

1. Emission is instantaneous: no measurable time lag, ever.
2. Stopping potential depends on frequency only, not intensity.
3. Saturation current is proportional to intensity, at fixed frequency.

Confirmed experimentally by the Davisson-Germer electron diffraction experiment (1927).