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All lesson and revision
materials for Physics for students, aged 14 to 19, are on our Llanishen Moodle
web site.
The PHYSICS DEPARTMENT moodle sites are updates weekly, to guide you through your Physics
GCSE, AS or A2 course.
Science's 10 Most Beautiful Physics Experiments
1. Double-slit electron diffraction
The French physicist Louis de Broglie proposed in 1924 that electrons and
other discrete bits of matter, which until then had been conceived only as
material particles, also have wave properties such as wavelength and
frequency. Later (1927) the wave nature of electrons was experimentally
established by C.J. Davisson and L.H. Germer in New York and by G.P. Thomson
in Aberdeen, Scot.
To explain the idea, to others and themselves, physicists often used a
thought experiment, in which Young's double-slit demonstration is repeated
with a beam of electrons instead of light. Obeying the laws of quantum
mechanics, the stream of particles would split in two, and the smaller
streams would interfere with each other, leaving the same kind of light- and
dark-striped pattern as was cast by light. Particles would act like waves.
According to an accompanying article in Physics World, by the magazine's
editor, Peter Rodgers, it wasn't until 1961 that someone (Claus Jönsson of
Tübingen) carried out the experiment in the real world.
2. Galileo's experiment on falling objects
In the late 1500's, everyone knew that heavy objects fall faster than
lighter ones. After all, Aristotle had said so. That an ancient Greek
scholar still held such sway was a sign of how far science had declined
during the dark ages.
Galileo Galilei, who held a chair in mathematics at the University of Pisa,
was impudent enough to question the common knowledge. The story has become
part of the folklore of science: he is reputed to have dropped two different
weights from the town's Leaning Tower showing that they landed at the same
time. His challenges to Aristotle may have cost Galileo his job, but he had
demonstrated the importance of taking nature, not human authority, as the
final arbiter in matters of science.
3. Millikan's oil-drop experiment
Oil-drop experiment was the first direct and compelling measurement of the
electric charge of a single electron. It was performed originally in 1909 by
the American physicist Robert A. Millikan. Using a perfume atomizer, he
sprayed tiny drops of oil into a transparent chamber. At the top and bottom
were metal plates hooked to a battery, making one positive (red in
animation) and the other negative (blue in animation). Since each droplet
picked up a slight charge of static electricity as it traveled through the
air, the speed of its motion could be controlled by altering the voltage on
the plates. When the space between the metal plates is ionized by radiation
(e.g., X rays), electrons from the air attach themselves to oil droplets,
causing them to acquire a negative charge. Millikan observed one drop after
another, varying the voltage and noting the effect. After many repetitions
he concluded that charge could only assume certain fixed values. The
smallest of these portions was none other than the charge of a single
electron.
4. Newton's decomposition of sunlight with a prism
Isaac Newton was born the year Galileo died. He graduated from Trinity
College, Cambridge, in 1665, then holed up at home for a couple of years
waiting out the plague. He had no trouble keeping himself occupied.
The common wisdom held that white light is the purest form (Aristotle again)
and that colored light must therefore have been altered somehow. To test
this hypothesis, Newton shined a beam of sunlight through a glass prism and
showed that it decomposed into a spectrum cast on the wall. People already
knew about rainbows, of course, but they were considered to be little more
than pretty aberrations. Actually, Newton concluded, it was these colors —
red, orange, yellow, green, blue, indigo, violet and the gradations in
between — that were fundamental. What seemed simple on the surface, a beam
of white light, was, if one looked deeper, beautifully complex.
5. Young's light-interference experiment
Newton wasn't always right. Through various arguments, he had moved the
scientific mainstream toward the conviction that light consists exclusively
of particles rather than waves. In 1803, Thomas Young, an English physician
and physicist, put the idea to a test. He cut a hole in a window shutter,
covered it with a thick piece of paper punctured with a tiny pinhole and
used a mirror to divert the thin beam that came shining through. Then he
took "a slip of a card, about one-thirtieth of an inch in breadth" and held
it edgewise in the path of the beam, dividing it in two. The result was a
shadow of alternating light and dark bands — a phenomenon that could be
explained if the two beams were interacting like waves. Bright bands
appeared where two crests overlapped, reinforcing each other; dark bands
marked where a crest lined up with a trough, neutralizing each other.
The demonstration was often repeated over the years using a card with two
holes to divide the beam. These so-called double-slit experiments became the
standard for determining wavelike motion — a fact that was to become
especially important a century later when quantum theory began.
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6. Cavendish's torsion-bar experiment
The experiment was performed in 1797–98 by the English scientist Henry
Cavendish. He followed a method prescribed and used apparatus built by his
countryman, the geologist John Michell, who had died in 1793. The
apparatus employed was a torsion balance, essentially a stretched wire
supporting spherical weights. Attraction between pairs of weights caused
the wire to twist slightly, which thus allowed the first calculation of
the value of the gravitational constant G. The experiment was popularly
known as weighing the Earth because determination of G permitted
calculation of the Earth's mass.
7. Eratosthenes' measurement of the Earth's circumference
At Syene (now Aswan), some 800 km (500 miles) southeast of Alexandria in
Egypt, the Sun's rays fall vertically at noon at the summer solstice.
Eratosthenes, who was born in c. 276 BC, noted that at Alexandria, at the
same date and time, sunlight fell at an angle of about 7° from the
vertical. He correctly assumed the Sun's distance to be very great; its
rays therefore are practically parallel when they reach the Earth. Given
estimates of the distance between the two cities, he was able to calculate
the circumference of the Earth. The exact length of the units (stadia) he
used is doubtful, and the accuracy of his result is therefore uncertain;
it may have varied by 0.5 to 17 percent from the value accepted by modern
astronomers.
8. Galileo's experiments with rolling balls down inclined planes
Galileo continued to refine his ideas about objects in motion. He took a
board 12 cubits long and half a cubit wide (about 20 feet by 10 inches)
and cut a groove, as straight and smooth as possible, down the center. He
inclined the plane and rolled brass balls down it, timing their descent
with a water clock — a large vessel that emptied through a thin tube into
a glass. After each run he would weigh the water that had flowed out — his
measurement of elapsed time — and compare it with the distance the ball
had traveled.
Aristotle would have predicted that the velocity of a rolling ball was
constant: double its time in transit and you would double the distance it
traversed. Galileo was able to show that the distance is actually
proportional to the square of the time: Double it and the ball would go
four times as far. The reason is that it is being constantly accelerated
by gravity.
9. Rutherford's discovery of the nucleus
When Ernest Rutherford was experimenting with radioactivity at the
University of Manchester in 1911, atoms were generally believed to consist
of large mushy blobs of positive electrical charge with electrons embedded
inside — the "plum pudding" model. But when he and his assistants fired
tiny positively charged projectiles, called alpha particles, at a thin
foil of gold, they were surprised that a tiny percentage of them came
bouncing back. It was as though bullets had ricocheted off Jell-O.
Rutherford calculated that actually atoms were not so mushy after all.
Most of the mass must be concentrated in a tiny core, now called the
nucleus, with the electrons hovering around it. With amendments from
quantum theory, this image of the atom persists today.
10. Foucault's pendulum
Last year when scientists mounted a pendulum above the South Pole and
watched it swing, they were replicating a celebrated demonstration
performed in Paris in 1851. Using a steel wire 220 feet long, the French
scientist Jean-Bernard-Léon Foucault suspended a 62-pound iron ball from
the dome of the Panthéon and set it in motion, rocking back and forth. To
mark its progress he attached a stylus to the ball and placed a ring of
damp sand on the floor below.
The audience watched in awe as the pendulum inexplicably appeared to
rotate, leaving a slightly different trace with each swing. Actually it
was the floor of the Panthéon that was slowly moving, and Foucault had
shown, more convincingly than ever, that the earth revolves on its axis.
At the latitude of Paris, the pendulum's path would complete a full
clockwise rotation every 30 hours; on the Southern Hemisphere it would
rotate counterclockwise, and on the Equator it wouldn't revolve at all. At
the South Pole, as the modern-day scientists confirmed, the period of
rotation is 24 hours.
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