The Science Book

111

See also: Christiaan Huygens 50–51 ■ Isaac Newton 62–69 ■
Léon Foucault 136–37 ■ Albert Einstein 214–21

A CENTURY OF PROGRESS

center, and Young wondered if
the rings might be caused by
interacting waves of light.

The double-slit experiment
Young made two slits in a piece of
cardboard and shone a beam of
light onto them. On a paper screen
placed behind the slits, the light
created a pattern that convinced
Young that it was waves. If light
were streams of particles, as
Newton said, there should simply
have been a strip of light directly
beyond each slit. Instead, Young
saw alternating bright and dark
bands, like a fuzzy bar code. He
argued that as light waves spread
out beyond the slits, they interact. If
two waves ripple up (peak) or down
(trough) at the same time, they make
a wave twice as big (constructive
interference)—creating the bright
bands. If one wave ripples up as the
other ripples down, they cancel each
other out (destructive interference)—
creating the dark bands. Young also
showed that different colors of light
create different interference
patterns. This demonstrated that
the color of light depends on its

wavelength. For a century, Young’s

double-slit experiment convinced

scientists that light is a wave, not

a particle. Then in 1905, Albert

Einstein showed that light also

behaves as if it were a stream of

particles—it can behave like a

wave and a particle. Such was the

simplicity of Young’s experiment

that, in 1961, German physicist

Claus Jönsson used it to show that

the subatomic particles electrons

produce similar interference, so that

they, too, must also be waves. ■

Scientific investigations are

a sort of warfare carried on

against all one’s contemporaries

and predecessors.

Thomas Young Thomas Young

The eldest of 10 children

raised by Quaker parents in

Somerset, England, Thomas

Young’s brilliant mind made

him a child prodigy, and he

was nicknamed the “Young

Phenomenon.” At 13, he could

read five languages fluently—

as an adult, he made the

first modern translation of

Egyptian hieroglyphics.

After medical training

in Scotland, Young set up

as a physician in London

in 1799, but he was a true

polymath who, in his spare

time, conducted inquiries

into everything from a

theory of musical tuning to

linguistics. He is most famous,

however, for his work on light.

In addition to establishing

the principle of interference

of light, he devised the first

modern scientific theory of

color vision, arguing that

we see colors as varying

proportions of the three main

colors: blue, red, and green.

Key works

1804 Experiments and

Calculations Relative to

Physical Optics

1807 Course of Lectures on

Natural Philosophy and the

Mechanical Arts

Here, light travels
through two slits in a piece
of card, and reaches a
screen. The light waves
passing through the slits
interfere. Where peaks
(yellow) intersect with
troughs (blue), there is
destructive interference.
Where peaks intersect
with peaks and troughs
with troughs, there is
constructive interference.

Light waves

Card with

two slits

Constructive interference

Destructive

interference

Screen

Pattern of light intensity