🌊 Module 2 · Lesson 1/3

🌊Waves or Particles? Both!

The double-slit experiment — “the heart of quantum mechanics”.

⏱️ 18 minStart

🎯 By the end you will…

  • ✓Recognise an interference pattern
  • ✓Explain wave–particle duality
  • ✓Use de Broglie's λ = h/p

Throw tennis balls at a wall with two slits and you get two stripes behind it. Send water waves through the same slits and you get many stripes: where two wave crests meet they add up, where a crest meets a trough they cancel. This striped signature is called interference, and only waves do it.

Now fire electrons — tiny particles — one at a time. Each one lands as a single dot. But after thousands of dots… the stripes of a wave appear!

🎛️Interactive experiment

Fire electrons one by one and watch the pattern build. Then switch on the “which-slit detector” — peeking destroys the stripes.

Each electron behaves like a wave passing through both slits at once and interfering with itself — then it lands as a particle in one spot. And if we set up a detector to find out which slit it used, the interference disappears. Richard Feynman called this “a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics.”

🧮Equation, decoded

de Broglie wavelength — every object has a wave

What each symbol means

  • “lambda”, the wavelength: distance between two crests
  • Planck's constant again — it appears everywhere in quantum physics
  • Momentum: mass × speed, the “oomph” of a moving thing

📖 Say it like a story

The more oomph something has, the shorter its wave. A thrown football has a huge p, so its wave is about 10⁻³⁴ m — hopelessly small to ever see. An electron has a tiny p, so its wave is about the size of an atom, and that's why electrons interfere.

✅ Check your understanding

Answer all questions to complete the lesson and earn XP.

  1. 1. What pattern do single electrons eventually build behind two slits?

  2. 2. What happens when we detect which slit each electron goes through?

  3. 3. According to λ = h/p, why don't we see a football's wave?

📚 Sources & further reading