The photoelectric effect is a recognition that different materials require different wavelengths of light to induce ionization. The key difference being that the very initial point of ionization begins at a fixed wavelength rather than how many photons were being shot at the material. There is a minimum threshold required to allow ionization.
This means that if you expose a material to light with too long a wavelength, no matter how many photons you irradiate this material with, it will not ionize. The material only heats up and that is it. One photon will not do it, neither will 10 or 100 or 1000 or even a million when their energy is below this threshold.
On the other hand, if you choose a small enough wavelength so that the photons have a sufficiently high energy, only one photon is required to ionize the material. When you hit this energy threshold, you basically get one ejected electron for each photon hitting the material. You effectively turn on and off the ionization by crossing that energy threshold known as the work function.
So this photoelectric effect demonstrates that if the incoming light has energy below the work function, the material irradiated does not ionize, it only heats up. If the light energy in each photon is larger than that of the material work function, you almost get a full ionization event with each photon. If the energy of each photon is substantially greater than the material work function, then the extra energy is carried off by the electron which was knocked off in the ionization process making it travel faster.
Most materials have this threshold energy starting in the ultraviolet region of light. It was Albert Einstein who first published that this was because a bound electron in a material has a certain amount of energy keeping it attracted to the nucleus that the ionizing photon must have at least that amount of energy in order to hope to knock it out and create an ionization event.
The real genius was in recognizing that this meant each photon had a fixed quantity of energy meaning the energy is "quantized". The photons couldn't add up their energy but could only deliver it in these fixed packets of quanta from which the term quantum mechanics gets its name (energy only comes in specific quantities of energy and is not continuous).
So not only was understanding the photoelectric effect the source of Albert Einstein's Nobel prize, it is also the reason solar panels work. The materials in solar panels are manufactured such that when a photon from the sun hits it, the photoelectric effect occurs and so ejects an electron as an ionization effect. This ionized electron is then captured in a separate conducting layer element in the matrix which then pulls the electron to electrical ground through an electronic circuit where it can do work.
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