Isaac Newton (1642–1727) was born in Woolsthorpe, Lincolnshire, in England, and studied at Trinity College, Cambridge. In 1665–1666, when the university closed because of the plague, he worked at home on mathematics, light and gravity. Later he became Lucasian Professor of Mathematics at Cambridge, President of the Royal Society, and Master of the Royal Mint.
Key idea
The prism
People had seen prisms make colors for centuries. Most thought the glass somehow added color to white light. Newton tested it. He let a beam of sunlight into a dark room through a small hole and sent it through a prism.
In a very dark Chamber, at a round Hole, about one third Part of an Inch broad, made in the Shut of a Window, I placed a Glass Prism, whereby the Beam of the Sun's Light, which came in at that Hole, might be refracted upwards toward the opposite Wall of the Chamber, and there form a colour'd Image of the Sun.
The colored image was long, not round: violet was bent more than red. Then came his key test, the experimentum crucis ("crucial experiment"). He picked out one color with a slit and sent it through a second prism. It bent again, but it stayed the same color. The prism wasn't making colors; it was sorting colors that were already in the white light. A lens could put them back together into white.
The reflecting telescope
Lenses act a little like prisms, so telescopes of the time made colored fringes around bright objects. Newton thought (wrongly, as it later turned out) that this could not be fixed in a lens. So he used a curved mirror instead, which reflects all colors the same way. Around 1668 he built a small reflecting telescope about six inches long, grinding the metal mirror himself. The Royal Society saw a copy in 1671 and elected him a Fellow in 1672. Most large research telescopes today are reflectors.
Gravity: from the apple to the Moon
Galileo had worked out how things fall on Earth. Newton asked whether the same pull reaches the Moon. If the Moon is about 60 times farther from Earth's center than we are, and gravity weakens with the square of the distance, then at the Moon it should be $60^2 = 3600$ times weaker. He checked that this weaker pull is just right to bend the Moon's path into its orbit. In the Principia (1687) he showed with a thought experiment how a cannonball could become a moon.
If a leaden ball projected from the top of a mountain by the force of gun-powder with a given velocity, and in a direction parallel to the horizon, is carried in a curve line to the distance of two miles before it falls to the ground; the same, if the resistance of the air was took away, with a double or decuple velocity, would fly twice or ten times as far.
Worked example: Weaker with distance
- $4^2 = 16$.
- So the pull is $\dfrac{1}{16}$ as strong.
Answer: 16 times weaker.
Newton gave credit to those before him. In a letter to Robert Hooke in February 1676 (1675 by the English calendar of the time) he wrote: "If I have seen further it is by standing on ye shoulders of Giants." (The saying is older than Newton, and the spelling in his letter is "sholders.") He and Hooke also argued bitterly, over light and over who first had the idea that gravity weakens with the square of distance.
Story or history?
Did an apple fall on Newton's head? No source says so. The apple story comes from Newton himself, late in life: he told his friend William Stukeley that seeing an apple fall in his garden at Woolsthorpe set him thinking about gravity. So the apple falling is Newton's own memory, told decades later; the bump on the head is a cartoon.
| When | What happened |
|---|---|
| 1642 | Born at Woolsthorpe, Lincolnshire, on Christmas Day (Old Style calendar) |
| 1661 | Enters Trinity College, Cambridge |
| 1665–1666 | Plague closes the university; Newton works at home on light, gravity and calculus |
| c. 1668 | Builds the first working reflecting telescope |
| 1672 | Elected to the Royal Society; publishes his theory of light and colors |
| 1687 | Principia: the laws of motion and universal gravitation |
| 1704 | Opticks: his experiments with light |
| 1727 | Dies in London |
Worked example: A long wait
- $1704 - 1666 = 38$.
Answer: 38 years.
What it led to
Newton's laws explain tides, planets, comets and the paths of spacecraft. His study of light founded the science of the spectrum, which today tells astronomers what stars are made of. And his reflecting telescope is the ancestor of the great mirrors in observatories.
Common mistake
Read the source on the Great Books Ladder
- Newton: The Principia: the laws of motion and the cannonball on the mountain
- Newton: Optics: the prism experiments in Newton's own words
Read more: Isaac Newton.