The Science Book

137

In Foucault’s experiment, the

speed of light was calculated from

the difference in angle as a beam of

light reflected back and forth between

a rotating mirror and a fixed mirror.

See also: Christiaan Huygens 50–51 ■ Ole Rømer 58–59 ■ Isaac Newton 62–69 ■ Thomas Young 110–11 ■
James Clerk Maxwell 180–85 ■ Albert Einstein 214–21 ■ Richard Feynman 272–73

A CENTURY OF PROGRESS

measurement was carried out by
French physicist Hippolyte Fizeau
in 1849. A beam of light was shone
through a gap between the teeth of
a rotating cogwheel. That light was
then reflected by a mirror that was
positioned 5 miles (8 km) away, and
passed back through the next gap
between the wheel’s teeth. Taking
the precise speed of rotation that
allowed this to happen, together
with time and distance, Fizeau
calculated the speed of light as
194,489 miles/s (313,000 km/s).

Contradicting Newton

In 1850, Fizeau collaborated with

fellow physicist León Foucault, who

adapted his apparatus—and made

it much smaller—by reflecting the

beam of light off a rotating mirror

instead of passing it through the

cogwheel. Light shining at the

rotating mirror would only be

reflected toward the distant mirror

when the rotating mirror was at

the correct angle. Light returning

from the fixed mirror was reflected

by the rotating mirror again, but

since this mirror had moved while

the light was traveling, it was not

reflected directly back toward the

source. The speed of light could

now be calculated from the angle

between the light going to and

from the rotating mirror and the

speed of rotation of the mirror.

The speed of light in water

could be measured by putting

a tube of water in the apparatus

between the rotating and

stationary mirrors. Using this

apparatus, Foucault established

that light traveled more slowly in

water than in air. As such, he

argued, light could not be a

particle, and the experiment was

viewed at the time as a refutation

of Newton’s theory of corpuscles.

Foucault refined his apparatus

further, and in 1862, measured the

speed of light in air as 185,168

miles/s (298,000 km/s)—remarkably

close to today’s value of 186,282

miles/s (299,792 km/s). ■

Léon Foucault Born in Paris, France, Léon

Foucault was educated mainly

at home before entering medical

school, where he studied under

the bacteriologist Alfred Donné.

Since he could not bear the sight

of blood, Foucault soon gave up

his studies, became Donné’s

laboratory assistant, and devised

a way of taking photographs

through a microscope—he later

teamed up with Hippolyte Fizeau

to take the first ever photograph of

the Sun. In addition to measuring

the speed of light, Foucault is best

known for providing experimental

evidence of Earth’s rotation, using

a pendulum in 1851 and later a

gyroscope. Although he had no

formal training in science, a post

was created for Foucault at the

Imperial Observatory in Paris.

He was also made a member of

several scientific societies, and

is one of 72 French scientists

named on the Eiffel Tower.

Key works

1851 Demonstration of Physical

Movement of Rotation of the

Earth by Means of the Pendulum

1853 On the Relative Velocities

of the Light in Air and in Water

Above all we must be

accurate, and it is an

obligation which we intend

to fulfill scrupulously.

Léon Foucault

Rotating

mirror

Fixed

mirror

Light

source

Reflected light

Tube of water (for the

speed of light in water)