Great IdeasNature and science
Light and color
What light is and how we see.
The question
What is light, and how do we see colors? Is light made of particles, of waves, or something stranger?
- 1 Aristotle 350 BC
- 2 René Descartes 1637
- 3 Christiaan Huygens 1690
- 4 Isaac Newton 1704
- 5 Albert Einstein 1916
- 6 Max Planck 1948
- Light as a wave
- Light as particles
- Something of both
How the books answer
Oldest first. Each stop is our own short summary of what the author argued; follow the link to the rung for the whole lesson.
c. 350–322 BC
Aristotle, On the Soul, and the short works on sense, memory, sleep and dreams
Aristotle says light is not a thing that travels but a state of a clear medium, like air or water, when it is lit by fire or the sun. Color moves the lit medium, and the medium moves the eye. Seeing needs something in between.
1637
René Descartes, Optics (published with the Discourse)
In the Optics printed with his Discourse, Descartes explains light as a pressure passed through a medium, like a blind man's stick feeling the ground. He works out the law of refraction for light bending into glass or water. He uses it to explain lenses and the rainbow.
1690
Christiaan Huygens, Treatise on Light
Light as a wave
Huygens argues that light is a wave spreading through a fine medium, like sound but far faster. Each point on a wave sends out its own little wave, and together they make the next wavefront. This explains reflection and refraction, and he notes that light's speed is great but finite.
“This, however, is quite another thing from being instantaneous, since there is all the difference between a finite thing and an infinite.”
1704
Isaac Newton, Optics
Light as particles
Newton splits sunlight with a prism and shows that white light is a mixture of colors, each bent by a different amount. At the end of the book he asks, as a question, whether light might be made of tiny particles. For a century his view was favored over Huygens'.
“Are not the Rays of Light very small Bodies emitted from shining Substances?”
1916 (English 1920)
Albert Einstein, Relativity: The Special and the General Theory
Einstein builds relativity on the principle that light's speed in empty space is the same for every observer, however they move. This simple rule changes our ideas of space and time. Light becomes the measuring stick of the universe.
“If the observer perceives the two flashes of lightning at the same time, then they are simultaneous.”
1948 (English 1949)
Max Planck, Scientific Autobiography and Other Papers
Something of both
Planck looks back on how, in 1900, he found that energy is given off in small packets called quanta. Einstein and others then showed that light itself behaves both like waves and like particles. Planck writes about how hard it was even for him to accept the new picture.
For young readers
Light lets us see. It travels super fast, from the sun to the earth in about eight minutes.
Newton found something amazing: white sunlight is really all the colors mixed together. A prism, or a glass of water in a sunny window, can split it into a rainbow. Try it! Scientists argued for years whether light is like tiny bullets or like waves on a pond. The answer turned out to be: a bit of both.
Talk about it
- Make a rainbow with a glass of water or a hose. Where do the colors come from?
- Why does a straw look bent in water?
- Huygens thought light was a wave, Newton thought particles. How could both be partly right?
- Why do you think we see lightning before we hear thunder?
Where else on the site
- Light and Color
- Refraction and Lenses
- Snell's Law
- Huygens' Principle
- Photons
- The Postulates of Relativity
Every rung tagged with this idea:
These pages are our own short synthesis, written for families. They were inspired by Mortimer J. Adler's Syntopicon, the index of great ideas in volumes 1 and 2 of Great Books of the Western World, but the questions, summaries and groupings are ours, not the Syntopicon's. Short excerpts come from public-domain translations, credited where they appear.