IB Physics Sub-topic E2 Notes

The photoelectric effect

Electromagnetic radiation is traditionally thought of as a wave. However, Einstein proposed their nature to be particular in his explanation of the photoelectric effect, the emission of electrons from a metal surface when UV light shines on it.

The process is as follows:

1. UV light with frequency f arrives in particles called photons. Their energy E is:

   $E = hf$

2. The minimum energy that electrons need to escape the energy is called the work function (Φ). Every electron absorbs a photon with a different frequency/energy and if the photon is below a threshold frequency (f₀), no electron is emitted.
3. If the energy is higher than the work function, the electron escapes and the remaining energy is converted into kinetic energy. The maximum kinetic energy (E_max) is:

   $E_{max} = hf - (\Phi + \epsilon)$

4. The number of electrons emitted depends on the UV light intensity.
5. The process with a reverse supply is known as the stopping potential experiment. In this, the voltage is increased until electrons are decelerated to a point of no emission, called the stopping potential (V_s) and proportional to the electron’s maximum kinetic energy. The equation is:

   $E_{k} = V_{s}e$

The graphs below show how the current varies with UV light’s intensity and frequency:

1. An increase an intensity releases more electrons, increasing maximum current.

2. An increase in frequency increases kinetic energy and stopping potential.