Physics

The photoelectric effect depends on photon energy, not intensity. Red light at 650 nm has too little energy per photon to overcome zinc's work function—no matter how many photons hit, individual photons can't knock electrons free. UV photons, though fewer in number, carry much higher energy ($E = hf$), so even a faint source can eject electrons. Intensity only determines how many photoelectrons are produced once the energy threshold is crossed.

The photoelectric effect depends on photon energy, not intensity. Red light at 650 nm has too little energy per photon to overcome zinc's work function—no matter how many photons hit, individual photons can't knock electrons free. UV photons, though fewer in number, carry much higher energy ($E = hf$), so even a faint source can eject electrons. Intensity only determines how many photoelectrons are produced once the energy threshold is crossed.