Simulate Einstein's photoelectric effect: shine light of adjustable frequency on a metal surface and measure kinetic energy of ejected electrons.
The photoelectric effect provided the decisive experimental evidence that light behaves as discrete particles (photons) rather than a purely continuous wave — Einstein's explanation of it, not his relativity work, earned him the Nobel Prize, and it remains a cornerstone AP Chemistry Unit 1 topic precisely because it cannot be explained without quantum theory.
When light of sufficient frequency strikes a metal surface, electrons are ejected — but classical wave theory predicted this should depend on light intensity (brighter light, more energetic electrons), and experiments showed the opposite: ejected electron kinetic energy depends only on light frequency, not intensity at all. Einstein's equation, KE = hν - φ, resolves this: each photon carries energy hν (h = Planck's constant), and φ (the work function) is the minimum energy needed to free an electron from that specific metal's surface. Any photon energy beyond φ becomes the ejected electron's kinetic energy.
Below the threshold frequency ν0 = φ/h, no electrons are ejected regardless of how intense the light is — even an enormously bright beam of low-frequency light simply cannot eject a single electron, because no individual photon carries enough energy, and photons don't combine their energy together to eject one electron collectively. Above threshold frequency, increasing intensity increases the number of electrons ejected per second (more photons arriving means more individual ejection events) but does not increase each electron's kinetic energy — intensity and photon energy are independent variables.
Stopping voltage provides the standard experimental measurement: an opposing electric field is increased until it just stops the highest-energy ejected electrons from reaching a detector, and Vs = KE/e directly gives the maximum kinetic energy from an easily measured voltage.
Different metals have different work functions (sodium and potassium have low work functions, ejecting electrons even with visible light; platinum and gold require higher-energy UV light) — a direct, measurable consequence of how tightly each metal's electrons are held.
This simulator lets you adjust light frequency and intensity on real metals from sodium through platinum, directly visualizing threshold frequency, ejected electron kinetic energy, and stopping voltage as Einstein's equation predicts them.