Quick questions on The photoelectric effect and photons - TCE Physics (Tasmania)

Einstein proposed that light energy is carried in discrete quanta, now called photons. Each photon has energy proportional to the frequency $f$ of the light:

Energy conservation for one photon ejecting one electron gives Einstein's photoelectric equation:

In practice the maximum kinetic energy is measured by applying a reverse voltage that just stops the most energetic electrons. At the stopping voltage $V_\text{stop}$ the work done against the field equals the maximum kinetic energy, so $E_{k,\text{max}} = eV_\text{stop}$. This gives a clean experimental route to $E_{k,\text{max}}$ in electronvolts, as used in the calcium question above.