the photoelectric effect
Shine light on a clean metal surface and, if the light is right, electrons jump off it. That is the photoelectric effect. Everyday image: it is like knocking marbles out of a shallow cup. A gentle nudge does nothing at all, but a hard enough tap pops a marble out. The puzzle physicists faced around 1900 was that the strength of the 'tap' depended on the colour of the light, not on its brightness, and no theory of light as a smooth wave could explain that.
Precisely, the effect obeys Einstein's photoelectric equation: K_max = h f - phi. Here K_max is the maximum kinetic energy of an ejected electron, f is the light's frequency, h is Planck's constant, and phi (the work function) is the minimum energy needed to free one electron from that particular metal. The key facts: the electron's energy depends on frequency (colour), not on intensity (brightness); and below a threshold frequency, f_0 = phi / h, no electrons come out at all, no matter how blindingly bright the light is. Einstein explained this in 1905 by saying light arrives in packets, photons, each carrying energy h f; one photon frees at most one electron, and only if that single packet is energetic enough.
Why it matters: this was the first hard proof that light behaves as particles as well as waves, and it won Einstein the 1921 Nobel Prize. It runs solar cells, light meters, and the sensors in digital cameras. Common misconception to correct: making the light brighter (more photons per second) gives you more ejected electrons, but it never gives each electron more energy, and below the threshold frequency brightness achieves nothing at all.
For a caesium surface, dim blue light frees electrons, but even the most dazzling red floodlight frees none, because red photons individually fall below the work function. That is the effect in a nutshell: colour matters, brightness does not decide whether it happens.
Colour (frequency), not brightness, decides if electrons pop out.
The electron's kinetic energy depends on frequency via E = h f and the work function, never on intensity; below the threshold frequency, no electrons emerge no matter how bright the light.