THE FUNDAMENTALS OF PHYSICS
ALAMY5 distance for the light to travel is less.
Hence it arrives relatively early.
Conversely, when we are travelling
away, the light has to travel further
and arrives relatively late. Rømer’s
measurements – along with his
discovery of the correlation with
Earth’s motion – caused him to be
credited with the discovery. In 1690,
Dutch mathematician Christiaan
Huygens used t his to estimate a speed
for light of around 220,000km/s, about
70 per cent of the modern value.
The next step in the story again
involves astronomy, in this instance
the aberration of light, which may be
illustrated by a familiar phenomenon:
keeping dry as you move through
falling rain. Rain that is falling
ver tically when you’re at rest appea rs
to be falling from a point in front ofyou as you walk forwards – you have
to tip you r umbrella to keep d r y. Walk
in the opposite direction and the
origin of the raindrops now also
appears to be in the opposite direction.
Now think of the falling rain as light
travelling from a distant star and your
motion being that of Earth through the
heavens. The apparent position of a
sta r va ries du ring t he yea r due to t his
phenomenon, known as aberration.
James Bradley, the Astronomer
Royal, discovered this phenomenon in- He made measu rements of a sta r
in the constellation Draco and found
that its position moved first south and
then north on a six-month cycle. The
motion was little more than 1/100th of
a degree, but this could be seen easily
enough with 18th-century equipment.
From this, Bradley deduced that light
travels about 10,200 times faster than
Earth in its orbit, 295,000km/s, an
estimate that is within about two per
cent of the modern value.
Back down to Earth
To determine high speed requires
either access to large a distance, as in
astronomy, or the ability to measure
very small time intervals. The French
physicist Hippolyte Louis Fizeau in
1849 found a way to do this on Earth.
Fizeau shone light between the teeth
of a rapidly rotating wheel. A mirror
five miles away reflected the light
back. If the light passed through a gap,
it would be seen, but if it hit a tooth
between gaps, darkness would ensue.
He varied the speed of rotation and
from this was able to determine how
long the light had taken to make the
round trip. Knowing the distance to
the remote mirror, he was able to infer
the speed, some 313,000km/s. In 1862,
Léon Foucault used a similar idea, but
with rotating mirrors to determine the
angle through which the light had
been deflected. He found a speed of
299,796k m/s, rema rkably close to t he
modern value of 299,792.46km/s.
In 1865, the Scottish physicist James
Clerk Maxwell published his work on
electromagnetic waves, in which light
is a wave of elect ric a nd magnetic
fields. In any electromagnetic wave,
an electric field disappears and a 5The Draco constellation. From
noting the movement of one of its
stars, 18th-century astronomer James
Bradley deduced a speed for light“The apparent
position of a
star varies
during the
year, due to a
phenomenon
known as
aberration”