--- title: "The Photoelectric Effect and the Work Function" description: "Electrons leave a metal surface only above a threshold frequency, however bright the light gets, which no wave picture accounts for. One photon frees one electron, giving hf equal to the work function" canonical: https://lightmysky.com/learn/science/the-photoelectric-effect-and-the-work-function-mt_ADQK7WB2JN source: https://lightmysky.com/learn/science/the-photoelectric-effect-and-the-work-function-mt_ADQK7WB2JN.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # The Photoelectric Effect and the Work Function Electrons leave a metal surface only above a threshold frequency, however bright the light gets, which no wave picture accounts for. One photon frees one electron, giving hf equal to the work function plus the greatest kinetic energy. Subject: Science · Area: Waves, Light & Sound · Ages 17 to 18 Page: https://lightmysky.com/learn/science/the-photoelectric-effect-and-the-work-function-mt_ADQK7WB2JN ## Ready when they can - Names the observations a wave model of light cannot explain - Uses the photoelectric equation to find a threshold frequency or a maximum kinetic energy - Explains why brighter light frees more electrons but not faster ones ## Lesson: When light knocks electrons loose Shine light on some metals and electrons pop out at once. A wave picture fails three tests. Dim blue light frees electrons instantly, while bright red light frees none at all, however long you wait. The top speed of the freed electrons depends on colour, not brightness. **Example.** Think of each photon as one coin. One photon meets one electron and pays an exit fee called the work function. What is left becomes the electron's top kinetic energy. A 6.0e-19 J photon hitting metal with a 4.0e-19 J work function frees an electron with 2.0e-19 J to spare. The split works both ways: photon energy equals exit fee plus leftover kinetic energy. Turn up the brightness without changing the colour and more photons arrive each second. More electrons leave each second, but each one still meets a photon with the same energy, so the top speed stays fixed. Change to a higher frequency instead and each photon carries more energy, so freed electrons can leave faster. **Tip.** When no electrons leave, check the frequency before the brightness. Below the threshold frequency the photons cannot pay the exit fee, so nothing escapes however bright the beam. A dimmer but bluer lamp is the fix, not a brighter red one. **Recap.** One photon frees one electron when its frequency clears the threshold, and brightness only sets how many leave. ## Practice 13 questions on this page, each with its working shown. ## Needs first - [Electric Potential and the Uniform Field Between Plates](https://lightmysky.com/learn/science/electric-potential-and-the-uniform-field-between-plates-mt_LKTfeYxNde) - [Photon Energy and the Electronvolt](https://lightmysky.com/learn/science/photon-energy-and-the-electronvolt-mt_UfmDhGRaYT) ## Opens up - [Energy Levels and Line Spectra](https://lightmysky.com/learn/science/energy-levels-and-line-spectra-mt_RlMCSDxE5q) - [de Broglie Waves and Electron Diffraction](https://lightmysky.com/learn/science/de-broglie-waves-and-electron-diffraction-mt_Y8NthsmPcv)