Complete each stage to unlock the next one.
You can aim a lighthouse beam — carrying billions of joules per hour — at a zinc plate and nothing happens. Not a single electron moves. Then you switch on a small UV lamp that carries a tiny fraction of that power, and electrons leap off the metal instantly. More energy, zero effect. Less energy, immediate effect. In 1887, Hertz discovered this contradiction and had no explanation. In 1905, a 26-year-old patent examiner named Albert Einstein solved it in a single paragraph — and that solution won him the Nobel Prize 16 years later. The answer demolished the idea that light is simply a wave.
You can aim a lighthouse beam — carrying billions of joules per hour — at a zinc plate and nothing happens. Not a single electron moves. Then you switch on a small UV lamp that carries a tiny fraction of that power, and electrons leap off the metal instantly. More energy, zero effect. Less energy, immediate effect. In 1887, Hertz discovered this contradiction and had no explanation. In 1905, a 26-year-old patent examiner named Albert Einstein solved it in a single paragraph — and that solution won him the Nobel Prize 16 years later. The answer demolished the idea that light is simply a wave.
Classical wave physics predicted that enough light intensity should always eventually eject electrons — just a matter of waiting. It was completely wrong. Einstein's insight: light does not arrive as a continuous wave you can pour onto a surface. It arrives as discrete packets — photons — each carrying energy E = hf. An electron either receives one photon with enough energy to escape, or it doesn't. Pouring in more red photons just delivers more underpowered packages: none of them unlock the door.
Light consists of photons, each carrying energy E = h\nu, where h = 6.63×10⁻³⁴ J·s (Planck's constant) and \nu is the frequency. To eject an electron from a metal, a photon must have energy greater than the work function \phi — the binding energy that holds an electron in the metal. Any energy above \phi becomes kinetic energy of the ejected electron. If h\nu < \phi, no electrons are ever ejected, regardless of intensity.
The threshold frequency \nu₀ is where KE = 0: h\nu₀ = \phi, so \nu₀ = \phi/h. Below \nu₀: no emission. At \nu₀: electrons barely escape with zero KE. Above \nu₀: KE_max = h\nu − \phi. Increasing intensity above threshold increases the NUMBER of ejected electrons (more photons → more collisions) but NOT their individual KE. This separation — frequency controls KE, intensity controls count — is impossible in classical wave theory and proved the quantum nature of light. In 1914 Millikan measured KE vs frequency for several metals, found a straight line with slope h for all of them, and confirmed h = 6.63×10⁻³⁴ J·s to better than 0.5%.