Electrons are ejected from a metal surface when light above a threshold frequency strikes it, supporting the photon concept.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
In the photoelectric effect, light striking a metal surface ejects electrons only if its frequency exceeds a threshold value, regardless of intensity. Brighter light above the threshold increases the number of electrons emitted but not their maximum kinetic energy. Below the threshold no electrons are ejected, however long or intense the illumination. Einstein explained this in 1905 by treating light as photons with energy h nu, so that the maximum kinetic energy is KE = h nu - phi, where phi is the work function. The effect is the basis of photocells and photodetectors.
Key ideas: Light on a metal ejects electrons only if frequency exceeds the threshold, whatever the intensity. Einstein's equation: h nu = work function + kinetic energy max, i.e. KE = h nu - phi. Intensity changes the number of electrons (current), not their kinetic energy. Stopping potential V_s satisfies e V_s = KE max.
Common mistakes and connections: Students think brighter light should eject electrons even at low frequency (the classical expectation), or that ejection is delayed. In fact it is instantaneous. A graph of KE versus nu is a straight line with slope h and intercept -phi. It supports the particle nature of light, linking to photons and de Broglie.
For sodium with work function about 2.3 eV, only light above a threshold frequency ejects electrons, and brighter light just gives more of them. The interactive lesson shows this as a live animation or chart.
Violet light ejects electrons from potassium, but intense red light below the threshold ejects none.
Electrons are ejected from a metal surface when light above a threshold frequency strikes it, supporting the photon concept.
Photoelectric Effect is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Electromagnetic Spectrum, Black Body Radiation, Planck's Quantum Theory.
The next topics in Atomic Structure & Periodicity are Threshold Frequency and Work Function.
Yes. The interactive lesson includes a self-check question on Photoelectric Effect, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Photoelectric Effect is topic 37 of 115 in Atomic Structure & Periodicity.