Photoelectric Effect
Light can knock electrons out of metal, but only if its frequency is high enough.
About this topic
When light shines on a metal, electrons can be ejected, but only if the frequency exceeds a threshold value, regardless of intensity. Each photon carries energy E = hf, and an electron escapes if hf is greater than the work function Φ; the extra becomes kinetic energy: KE = hf − Φ.
Classical wave theory predicted that brighter light of any colour should eventually eject electrons, but experiments showed no emission below the threshold frequency, however bright, and instant emission above it even in dim light. Einstein explained it in 1905 with light quanta, work for which he won the Nobel Prize, and it supported the particle picture of light. Higher intensity means more photons, so more electrons, but not faster ones.
Worked example: for a metal with work function 2.0 eV, blue light of 3.0 eV photons ejects electrons with maximum KE of 1.0 eV, while red light with 1.8 eV photons ejects none at all, even from a powerful lamp. Planck's constant is h = 6.63 × 10⁻³⁴ J·s, and 1 eV is 1.6 × 10⁻¹⁹ J.
Applications include solar cells, light sensors, automatic doors, digital camera sensors and night vision devices. A misconception is that brighter light gives electrons more energy; only higher frequency does. Another is that light is only a particle; it behaves as both a wave and particle depending on the experiment.
Key ideas
- Energy of a photon = hf
- Threshold frequency needed
- Evidence for light as particles
Quick check
Use cases: where Photoelectric Effect helps
- Homework: review the key idea behind Photoelectric Effect before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Photoelectric Effect without notes.
- Visual learning: play with the interactive lab to see how Photoelectric Effect changes when you adjust its inputs.
- Teaching a friend: explain Photoelectric Effect in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Photoelectric Effect?
Light can knock electrons out of metal, but only if its frequency is high enough.
What will I learn in Photoelectric Effect?
Energy of a photon = hf. Threshold frequency needed. Evidence for light as particles.
Is Photoelectric Effect part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Photoelectric Effect lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Photoelectric Effect?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.