Cosmos Notebook

Learning paths · Path

Quantum from your kitchen

Measure the speed of light and Planck's constant — two of nature's fundamental numbers — with a microwave, a laser pointer and some LEDs.

QuantumBeginner8–10 weeks · 2–3 hours a week3 stages · 7 steps + 1 optional

Is light a wave or a stream of particles? By the end of this path you'll have seen it behave as both, and measured the two numbers that tie those pictures together: the speed of light, c, and Planck's constant, h.

Safety: never point a laser pointer at anyone's eyes, and never look into the beam or its reflection off something shiny.

  1. Weeks 1–2

    What is light?

    Start with the big picture, then measure the fastest thing there is using a bar of chocolate.

    1. ReadSeven Brief Lessons on Physics — the lesson on quanta
    2. DoMeasure the speed of light with chocolate

    Checkpoint Your microwave's frequency is printed on a label on the back (usually 2,450 MHz). Explain to a friend how two melted spots half a wavelength apart give you the speed of light.

  2. Weeks 3–5

    Light is a wave

    Split light into colours, then make it interfere with itself. Interference is the fingerprint of a wave.

    1. DoBuild a CD spectroscope
    2. DoThe double-slit experiment at home
    3. UsePhET Interactive Simulations — try the "Wave Interference" simulation before the real thing

    Checkpoint Why do the bright bands from the double slit spread further apart when the gap between the slits gets narrower?

  3. Weeks 6–10

    Light is also a particle

    Light also arrives in lumps of energy called photons. Measure the size of those lumps with LEDs, then meet the strangest part of quantum mechanics: measuring something changes it.

    1. ReadQuantum: A Guide for the Perplexed — the chapters on photons and the double slit
    2. DoFind Planck's constant with LEDs
    3. DoThe three-polariser puzzle
    4. UsePhET Interactive Simulations — the "Photoelectric Effect" simulation Optional
    5. ReadThe Theoretical Minimum: Quantum Mechanics — when you want the real maths; Susskind's free lectures follow the book Optional

    Checkpoint A blue LED needs a higher voltage to switch on than a red one. Explain why, using the energy of a single photon, E = hf.

Method

How every project runs

  1. Learn just enough. One short video or chapter on the idea.
  2. Predict. Write down what you expect before measuring.
  3. Do it. Record everything, including the messy bits.
  4. Explain it. A one-page write-up for a friend — share it with the community if you like.
  5. Follow the questions. They become the next project.