Photoelectric effect — common mistakes

Always remember to convert units, especially electronvolts to Joules (and vice-versa) using 1 eV = 1.60 × 10⁻¹⁹ J. Questions often involve mixed units, so pay close attention to what the question is asking for! Also, be careful with wavelength units; they are often given in nanometres (nm) and must be converted to metres (m) for calculations with 'c' and 'h'.

Because electrons need a minimum amount of energy (the work function, Φ) to escape the metal. Since photon energy E = hf, only photons with a frequency equal to or greater than the threshold frequency (f₀) will have enough individual energy to eject an electron. Below f₀, photons simply don't have enough energy, regardless of how many there are.

The work function (Φ) is the minimum energy an electron needs to absorb to escape the metal surface, measured in Joules or electronvolts. The threshold frequency (f₀) is the minimum frequency of incident light required for photons to provide this specific energy, related by the equation Φ = hf₀.

Above the threshold frequency, increasing the intensity of light means more photons are striking the metal surface per second. Since each photon can potentially eject one electron, a higher intensity leads to a greater number of electrons being emitted per second, resulting in a larger photocurrent.

Stopping potential (V_s) is the reverse voltage needed to stop the fastest photoelectrons. The work done by this potential (eV_s) must equal the maximum kinetic energy of the electrons. Therefore, KE_max = eV_s. This provides a direct experimental method for measuring the maximum kinetic energy.