The photoelectric effect is the ability to knock electrons off of the atoms on the surface of a solid by shining a light at it. For some colors of light, it doesn’t matter how bright and intense the light is, no electrons separate from the solid. For other colors of light, it doesn’t matter how dim and weak the light is, electrons do separate from the solid. The dividing line between the colors that do and the colors that don’t is a certain wavelength of light, the one where each particle of light has at least enough energy to rio an electron away from an atom. If each particle of light has less energy than that, then no matter how many particles there are, they can’t separate electrons from their atoms. If each particle of light has more energy than that, than it doesn’t matter how few of them there are. A single particle can rip and electron away from the electron as it comes flying away from the surface.