Lesson 06 · Optics & Light
Newton's Dispersive Prism
In 1666, a 23-year-old Isaac Newton bought a glass prism at a fair, darkened his room, let in a narrow beam of sunlight — and proved something nobody believed: white light isn't pure. It's every colour at once.
Key idea
A prism does not create colour. It separates wavelengths already present inside white light.
- Explain why violet bends more than red in glass.
- Describe Newton's two-prism proof of white light.
- Connect dispersion to modern spectroscopy.
The Visible Spectrum
Newton named seven colours to match the seven notes of the musical scale — though the spectrum is actually continuous.
Interactive Prism Simulator
Adjust the beam angle and prism rotation to see how different angles of incidence change the dispersion. The spectrum fans out as white light slows down inside the glass — each wavelength by a different amount.
Toggle the second prism to recombine the spectrum back into white light — Newton's crucial proof that the prism doesn't add colour, it only separates it. Use the Parameters panel to adjust the simulation.
What Newton Proved
Before 1666, the accepted view — held since Aristotle — was that colour was a modification of white light. A prism was thought to "stain" pure white light with colour the way a dye stains cloth.
"The most surprising and wonderful composition was that of whiteness. I had never seen any author write of whiteness being a mixture of all colours."
— Isaac Newton, Opticks, 1704
Newton's genius was his crucial experiment (experimentum crucis): he used a second prism, inverted, to recombine the dispersed spectrum back into white light. If the prism had been adding colour, a second prism couldn't remove it. Recombination proved the colours were already there — hidden inside white light all along.
How Dispersion Works
When light enters glass at an angle, it slows down. Different wavelengths slow by different amounts — this is called dispersion.
Violet bends most
Shorter wavelengths interact more strongly with the glass — violet slows down the most and bends at the steepest angle.
Red bends least
Longer wavelengths pass through more easily — red light slows the least and exits closest to its original direction.
Snell's Law
The bending is governed by n = c/v — the refractive index. For glass, n ≈ 1.52 for red and ≈ 1.54 for violet, causing the spread.
Newton's Annus Mirabilis — 1666
The year 1666 is called Newton's annus mirabilis (year of wonders). Forced home from Cambridge by the Great Plague, Newton — aged 23 — made three of the most important discoveries in the history of science in a single year.
Born in Woolsthorpe, Lincolnshire
Born premature on Christmas Day. His father had died before his birth. He was small enough to fit in a quart pot, by family legend.
Enters Trinity College, Cambridge
Works as a subsizar — waiting on wealthier students — to pay his fees. Secretly reads Descartes, Kepler, and Galileo outside the official curriculum.
Great Plague closes Cambridge
Newton retreats to Woolsthorpe for 18 months. Freed from lectures, he begins his most productive period of thinking.
Annus Mirabilis — three discoveries
① Prism experiment proves white light is a spectrum mix. ② Develops calculus (independently from Leibniz). ③ Begins formulating universal gravitation after observing a falling apple.
Builds the first reflecting telescope
Using mirrors instead of lenses, Newton eliminates chromatic aberration — the rainbow fringing that plagues refracting telescopes because of dispersion.
Publishes Opticks
His complete theory of light and colour, written in accessible English (not Latin). Describes the prism experiment, colour mixing, and the nature of the spectrum in full detail.
Why This Matters Today
Newton's prism experiment laid the foundation for the entire science of spectroscopy — one of the most powerful tools in modern science and technology.
Rainbows
Raindrops act like tiny prisms — sunlight enters, reflects off the inside, and exits dispersed. Newton's theory explains exactly why red is at the top.
Stellar Spectroscopy
Spreading starlight into a spectrum reveals which elements are present. We know what stars are made of — from 100 trillion km away — because of Newton's prism.
Camera Sensors & Screens
RGB colour in every camera and screen is built on the understanding that white light (and white pixels) is a mixture of red, green, and blue.
Reflecting Telescopes
Newton invented the reflecting telescope to escape chromatic aberration. Every modern observatory telescope — including Hubble — uses mirrors for this reason.
Medical Diagnostics
Spectroscopy identifies molecules by the light they absorb. MRI contrast agents, blood oxygen sensors, and lab tests all descend from Newton's experiment.
Hubble's Discovery
Edwin Hubble used spectroscopy to show galaxies are moving away from us — their light is red-shifted. This proved the universe is expanding.
Check Your Understanding
Answer each question, then read the explanation.
1. Before Newton's experiment, what did scientists believe about white light and colour?
2. Why did Newton use a second, inverted prism in his experiment?
3. Which colour of light bends the most when passing through a prism, and why?
4. Personal Reflection (open-ended)
Newton made his discovery during a pandemic lockdown, aged 23, with a prism bought at a fair. What does that tell you about where and how scientific breakthroughs happen? Can you think of another example?
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