Learning journey · stage 3

What is light made of?

Around 1900 it seemed settled: light is a wave. In 1905 a 26-year-old clerk at the patent office in Bern claimed that light comes in separate packets. Three stations: at each one you commit to an answer first, then check it in the experiment.

Station 1 of 3

The photoelectric effect

Light can knock electrons out of a metal. But something happened that did not fit the idea of light as a wave. In March 1905 Einstein proposed an explanation: light consists of separate packets of energy, light quanta. Today we call them photons.

The stations need JavaScript. The experiments themselves are also in the chapters Quantum physics, E = mc² and Miracle year.

Interactive · photoelectric effect

Light meets metal

2.07 eV
Energy of one light quantum · needed to knock one out: 2.30 eV
0
Electrons per second (model)
0.00 eV
Kinetic energy of the fastest electrons

No electrons: each light quantum has only 2.07 eV, but 2.3 eV are needed. More brightness does not change that.

Model of a sodium-like metal: work function 2.3 eV, threshold at 5.6 × 10¹⁴ Hz (green light, about 540 nm). Light quanta and electrons are drawn heavily thinned out and slowed down. In the model every light quantum above the threshold frees an electron; in reality only a small fraction do.

Station 2 of 3

E = mc²

In September 1905 Einstein sent in a three-page addendum: if a body gives off energy, it becomes lighter. Mass and energy are two sides of the same thing. This became the most famous formula in physics.

The stations need JavaScript. The experiments themselves are also in the chapters Quantum physics, E = mc² and Miracle year.

Interactive · E = mc²

Energy calculator

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Comparisons: a US home uses about 10,800 kWh of electricity a year; the Hiroshima bomb released about 6.3 × 10¹³ joules. Mass can hardly ever be turned completely into energy – nuclear fusion in the Sun converts about 0.7 percent.

Station 3 of 3

The miracle year 1905

Light quanta and E = mc² come from the same year. In 1905 Einstein was 26, worked as an examiner at the patent office in Bern and, on the side, wrote five papers, including his doctoral thesis. Today this year is called his miracle year.

The stations need JavaScript. The experiments themselves are also in the chapters Quantum physics, E = mc² and Miracle year.

Interactive · Calendar

The year 1905, day by day

Pick a paper: tap its row in the calendar.

  • Dated / finished
  • Submitted in Zurich
  • Received by the Annalen
  • Published
  • 14 March: Einstein's 26th birthday

1 Light quanta

„Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt“

Translation: On a Heuristic Point of View Concerning the Production and Transformation of Light

Annalen der Physik 17, S. 132–148

  • Dated / finished: 17 March 1905 Einstein was 26 years and 3 days old.
  • Received by the Annalen: 18 March 1905 Einstein was 26 years and 4 days old.
  • Published: 9 June 1905 Einstein was 26 years and 87 days old.

What it changed: Light comes in packets of energy, light quanta — this explained the photoelectric effect, helped found quantum theory and won him the Nobel Prize.

Where the sources differ: German Wikipedia gives 17 March: that is the date at the end of the manuscript (“Bern, den 17. März 1905”). According to the journal it was received on 18 March.

Read more: Quanta & the Great Debate

Sources: Annalen der Physik 17 (1905), S. 132–148 (Faksimile) · en.wikipedia: Annus mirabilis papers · de.wikipedia: Annus mirabilis · de.wikipedia: Albert Einstein

Sources:
  1. Wikipedia: Photoelectric effect
  2. Wikipedia: Robert Millikan
  3. Wikipedia: Mass–energy equivalence
  4. Wikipedia: Sun – 0.7 percent and 4.26 billion kilograms per second
  5. Wikipedia: Annus mirabilis papers
  6. Nobelprize.org: The Nobel Prize in Physics 1921