= Stopping potential
{title2=$eV_s=K_{\max}$}
A <stopping potential> $V_s\geq0$ is the magnitude of the retarding <electric potential difference> just sufficient to stop collection of even the fastest relevant <Electrons> in an ideal collection geometry. Since the <Electron>'s <electric charge> is $-e$, the positive <energy> barrier is $eV_s$. After the <photoelectric contact-potential correction>,
$$
V_s=\frac{h}{e}\nu-\frac{\Phi}{e}.
$$
The maximum here is an emission <energy> edge, not the mean <kinetic energy>. In an ideal planar retarding geometry, transmission instead selects the component normal to the plates; identifying its endpoint with the full edge requires collection of trajectories that attain that edge in the relevant direction.
To make a complete illustrative curve, choose a uniform distribution of outward normal <kinetic energy> $K_\perp$ on $0\leq K_\perp\leq K_{\max}$, complete collection at zero barrier, and zero background. This is a deliberately chosen distribution, not a prediction of the <Einstein photoelectric equation>. For an effective retarding barrier $eV_r$, the collected fraction is
$$
F(V_r)=\begin{cases}
1,&V_r\leq0,\\
1-V_r/V_s,&0<V_r<V_s,\\
0,&V_r\geq V_s.
\end{cases}
$$
Integrating that uniform <probability density> above $eV_r$ proves the middle line. Thus $I(V_r)=I_{\rm sat}F(V_r)$ has a common endpoint when incident <light intensity> changes, even though its vertical scale changes. Real emission distributions need not give straight segments.
\Image[/photoelectric-frequency-and-intensity.png]
{title=<Frequency> sets the emission <energy> edge; <light intensity> sets a collection scale}
{description=Original chosen single-<photon> model with <work function> $2.30\,\mathrm{eV}$. Left: the allowed upper <kinetic energy> edge begins at the <photoelectric threshold frequency>; the dotted continuation is an unavailable single-<photon> channel. Right: the chosen uniform-normal-energy collection model at $8.00\times10^{14}\,\mathrm{Hz}$, with $I_{\rm sat}=e\eta P/(h\nu)$, $\eta=0.10$, and incident powers $1$ and $2\,\mathrm{\mu W}$. The shared <stopping potential> is about $1.009\,\mathrm V$. No points or curves are measured data.}
{source=photoelectric-frequency-and-intensity.py}
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