Light shining on a metal kicks electrons out — but only if the photons are energetic enough. By turning the bias voltage on the collector plate, you control which of those electrons make it across the gap. The current you measure is one of the cleanest pieces of evidence that light comes in discrete packets.
Two metal plates sit inside an evacuated tube. Light of frequency \(f\) shines on the left plate (the cathode K). Each metal has its own work function \(\Phi\) — the minimum energy needed to pull an electron free of the surface — which you can change with the material buttons below. If a single photon's energy \(hf\) beats the work function, it can liberate an electron from the surface. Einstein's photoelectric equation says the maximum kinetic energy of those electrons is
$$ KE_\text{max} \;=\; hf - \Phi. $$The other plate (the anode A) is held at a voltage \(V\) relative to K. When \(V > 0\), A is positive — every emitted electron is pulled across, and the current saturates. When \(V < 0\), A pushes electrons back. Only those with enough kinetic energy still reach. The "stopping voltage" \(V_\text{stop}\) is the magnitude of \(V\) at which even the fastest electron is just barely turned around:
$$ e\, V_\text{stop} \;=\; KE_\text{max} \;=\; hf - \Phi. $$Two famously quantum features fall out of this:
Einstein's equation \(KE_\text{max} = hf - \Phi\) gives a straight line. Slope is Planck's constant \(h\) (same for every material). x-intercept is the threshold frequency \(f_0 = \Phi/h\). Switching materials slides the line horizontally without changing its slope.
With \(f\) above threshold and \(V\) above the stopping voltage, current grows linearly with intensity — double the photons, double the electrons. Below threshold (or below \(-V_\text{stop}\)), the line collapses onto the x-axis no matter how bright the light. That intensity-independence of the onset is the classical surprise.
The experiment above forces us to treat light as quanta — discrete packets of energy \(hf\). But light also shows unambiguous wave behavior in other experiments, most famously double-slit interference. Both pictures are needed, and which one shows up depends on what you actually measure.
Why a wave? Send light through two narrow slits and let it fall on a screen beyond. You don't see two bright patches behind the slits (as you would for classical bullets) — you see alternating bright and dark fringes. The only way to get that pattern is if light at each slit is a wave that spreads out, and the two spreading waves add constructively (bright) or destructively (dark) on the screen. The fringe spacing \(\Delta y = \lambda D / d\) directly encodes the wavelength — wave behavior, through and through.
Why a particle? The photoelectric effect above. Dimming the light doesn't slow ejection down — it just makes ejections rarer. Below the threshold frequency, no amount of classical wave energy frees an electron, but one single photon with \(hf \geq \Phi\) does the job instantly. Energy is delivered in indivisible lumps, one photon at a time.
The reconciliation is quantum mechanics: light is described by a wave function whose squared amplitude gives the probability of detecting a photon at a given place. In the double slit, that wave passes through both slits and interferes with itself; photons still arrive one-by-one on the screen, but the distribution of where they land follows the bright-and-dark fringe pattern. The wave is the rulebook; the particle is the event.
A plane wave hits a barrier with two slits. Each slit becomes a new source of circular waves (Huygens' principle), and the two sets of waves overlap in the region beyond. Where crest meets crest, they reinforce (bright); where crest meets trough, they cancel (dark). Watching the right-hand screen, you get a stable pattern of fringes — the time-average of constantly interfering crests and troughs. A stream of classical particles could never produce that pattern.
The same light source now fires individual photons at a metal. Each photon is a discrete packet of energy \(hf\). If \(hf \geq \Phi\), one photon immediately frees one electron — no waiting for enough wave energy to accumulate on the surface. Dim the source until only one photon arrives at a time and you still see one-for-one ejections. That instantaneous, quantized delivery of energy is what waves alone cannot explain.