The Science Book

136

LIGHT TRAVELS MORE

SLOWLY IN WATER

THAN IN AIR

LÉON FOUCAULT (1819–1868)

I

n the 17th century, scientists

began to investigate light,

and whether it had a finite,

measurable speed. In 1690,

Christiaan Huygens published his

theory that light is a pressure wave,

moving in a mysterious fluid called

ether. Huygens thought of light as

a longitudinal wave, and predicted

that the wave would travel more

slowly through glass or water than

through air. In 1704, Isaac Newton

published his theory of light

as a stream of “corpuscles,” or

particles. Newton’s explanation

for refraction—the bending of a

beam of light as it passes from one

transparent material to another—

assumed that light travels faster

after it passes from air into water.

Estimates for the speed of light

relied on astronomical phenomena,

showing how fast light travels

through space. The first terrestrial

IN CONTEXT

BRANCH

Physics

BEFORE

1676 Ole Rømer makes the

first successful estimate of the

speed of light, using eclipses

of Io, one of Jupiter’s moons.

1690 Christiaan Huygens

publishes his Treatise on

Light, in which he proposes

that light is a type of wave.

1704 Isaac Newton’s Opticks

suggests that light is a stream

of “corpuscles.”

AFTER

1864 James Clerk Maxwell

realizes that the speed of

electromagnetic waves is so

nearly the same as the speed

of light that light must be a

form of electromagnetic wave.

1879–83 German-born US

physicist Albert Michelson

refines Foucault’s method and

obtains a measurement for the

speed of light (through air) that

is very close to today’s value.

Therefore, light must

travel in waves.

Foucault found

that light travels

more slowly in

water than in air.

Newton thought

light particles would

speed up going from air

to water, while Huygens

thought waves would

slow down.

Is light a stream

of particles or a wave?

Whichever it is,

light takes time

to travel.