Quantum Chemistry || Lec # 2 || Photoelectric Effect || Dr. Rizwana

Light’s particle-like behavior is used to explain the photoelectric effect: when photons hit a metal surface, they can eject electrons only if the photons carry enough energy. Albert Einstein’s 1905 idea of light as discrete packets of energy—photons—links directly to what happens at the metal surface: each photon can transfer its energy to a single electron, potentially knocking it free.

In the photoelectric effect, metals contain free electrons near their outer surface. Incoming photons interact with these electrons, and if a photon’s energy exceeds the energy barrier created by the electron’s binding in the metal, the electron escapes. That binding energy is described as the work function: every metal has its own characteristic work function, representing the minimum energy required to release an electron from the surface. The threshold frequency is the specific frequency of incident light needed to supply at least that work function. For example, sodium’s work function is given as 2.28, while zinc’s work function is listed as 4.31; different metals therefore require different threshold frequencies.

A practical way to observe the effect uses a setup with a cathode (the metal plate) and an anode, plus an ammeter to measure current. When incident radiation strikes the cathode, electrons are emitted and move toward the anode, causing current to flow that the ammeter records. The behavior depends on two key properties of the incident light: frequency and intensity.

Frequency controls the energy per photon. Once the incident frequency reaches the threshold frequency, additional frequency increases the kinetic energy of the ejected electrons. In the lecture’s framing, the kinetic energy (and thus the speed) of emitted electrons is directly proportional to the frequency of the incident light, after accounting for the work function barrier. Below the threshold frequency, electrons are not emitted because photons lack sufficient energy.

Intensity controls how many photons arrive per unit time. Increasing intensity increases the number of photons striking the surface, which increases the number of electrons ejected. As a result, the current rises with intensity because more electrons contribute to the measured flow. Putting it together: electron speed and kinetic energy track frequency, while the number of emitted electrons—and therefore the current—tracks intensity.

Overall, the photoelectric effect ties measurable electrical current to photon energy and photon flux, reinforcing the idea that light energy comes in discrete quanta rather than being spread continuously. The lecture closes by pointing to the next topic—line spectra of elements—after this foundational photoelectric description.