The photoelectric effect. Photons
The external photoelectric effect is the emission of electrons from a substance under the action of light. Experiments show: the saturation photocurrent is directly proportional to the light flux; the maximum kinetic energy of photoelectrons depends not on intensity but linearly on frequency (E_k max = eU_stop); each substance has a minimum frequency ν_min below which no effect occurs however strong the light is; the effect is inertialess. Wave theory could not explain this, but Einstein (1905) treated light as a stream of photons, each giving its whole energy hν to one electron: hν = A + E_k max, where A is the work function. Hence the red limit is ν_min = A/h, λ₀ = hc/A; stronger light means more photons and a larger current, but the energy of each photon is unchanged. The slope of the graph U_stop(ν) equals h/e, which allows h to be measured. In the internal photoeffect electrons in a semiconductor are freed by light and the conductivity inside the material increases (photoresistor).
Look at a calculator or watch with a solar cell: cover it with your hand and it stops working, an example of the internal photoeffect. Never look directly at the Sun and do not experiment with lasers or ultraviolet lamps.