Slide one wavelength from 100 to 400 nm and three things move together: the photon's energy, which of the three UV bands the WHO convention puts it in, and whether it carries enough to free an electron from the metal you pick. The headline is a threshold: at 300 nm zinc holds on, at 288.3 nm it lets go, and no visible photon — 3.100 eV at most — will ever do it. E = hc / lambda, with hc = 1239.842 eV·nm.