110
THE EXPERIMENTS
MAY BE REPEATED
WITH GREAT EASE
WHEN THE SUN SHINES
THOMAS YOUNG (1773–1829)
A
t the turn of the 19th
century, scientific opinion
was divided over the
question of the nature of light. Isaac
Newton had argued that a beam
of light is made of countless,
tiny, fast-moving “corpuscles”
(particles). If light consists of these
bulletlike corpuscles, he said, this
would explain why light travels in
straight lines and casts shadows.
But Newton’s corpuscles did not
explain why light refracts (bends
when it enters glass) or splits into
the colors of the rainbow—also
an effect of refraction. Christiaan
Huygens had argued that light
comprises not particles, but waves.
If light travels as waves, Huygens
said, it is easy to explain these
effects. However, Newton’s stature
was such that most scientists
backed the particle theory.
Then, in 1801, British physician
and physicist Thomas Young hit on
a design for a simple yet ingenious
experiment that would, he believed,
settle the question one way or the
other. The idea began when Young
was looking at the patterns of
light made by a candle shining
through a mist of fine water
droplets. The pattern showed
colored rings around a brightIN CONTEXT
BRANCH
PhysicsBEFORE
1678 Christiaan Huygens first
proposes that light travels as
waves. He publishes his
Treatise on Light in 1690.1704 In his book Opticks,
Isaac Newton suggests that
light comprises streams of
particles, or “corpuscles.”AFTER
1905 Albert Einstein argues
that light must be thought
of as particles, later called
photons, as well as waves.1916 US physicist Robert
Andrews Millikan proves
Einstein correct through
experiment.1961 Claus Jönsson repeats
Young’s double-slit experiment
with electrons, and shows
that, like light, they can
behave as waves as well
as particles.Shine a light through
two adjacent slits onto a
screen. Two pools of light
should be seen on
the screen.But instead, it creates
interfering patterns of
light and dark, just as water
waves would if water flowed
through two slits.If light is made of particles
that travel in straight
lines, then this can be proved
in a simple experiment...Light must travel as waves.