🌅 Module 1 · Lesson 3/3

💡Light Is Made of Particles

The photoelectric puzzle that won Einstein his Nobel Prize.

⏱️ 15 minStart

🎯 By the end you will…

  • ✓Describe the photoelectric effect
  • ✓Explain why colour matters more than brightness
  • ✓Use the equation KE = hf − φ

Shine light on a clean metal plate and sometimes electrons jump out. That is the photoelectric effect. The weird part: a dim violet lamp kicks out electrons instantly, while a blindingly bright red lamp kicks out none at all — no matter how long you wait.

If light were only a smooth wave, more brightness should mean more energy and eventually knock electrons loose. It doesn't.

🎛️Interactive experiment

Pick a colour and a brightness. Below the metal's threshold, nothing happens. Above it, each photon frees one electron — brighter light frees more, bluer light makes them faster.

In 1905, a 26-year-old patent clerk named Albert Einstein took Planck's packets seriously: light itself travels as a stream of particles, later called photons. One photon hits one electron. If that single photon doesn't carry enough energy, it doesn't matter how many of them arrive.

🧮Equation, decoded

Einstein's photoelectric equation

What each symbol means

  • The top speed-energy (kinetic energy) of an escaping electron
  • The energy delivered by one photon (from Planck!)
  • “phi”, the work function: the exit fee the metal charges each electron

📖 Say it like a story

Think of a nightclub with a cover charge φ. A photon hands its electron hf coins. The electron pays φ at the door and leaves with whatever is left as speed. If hf is less than φ, it can't pay — it stays inside.

🌍Where you meet it

☀️

Solar panels

Photons free electrons in silicon to make electric current.

📷

Camera sensors

Each pixel counts the electrons knocked loose by incoming light.

🚪

Light sensors

Automatic doors and street lights react to photons.

✅ Check your understanding

Answer all questions to complete the lesson and earn XP.

  1. 1. A very bright red light ejects no electrons from a metal. What will?

  2. 2. What does φ (phi) represent?

  3. 3. Einstein's Nobel Prize was awarded for…

📚 Sources & further reading