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Italian scientist Galileo Galilei spent a lot of his time trying to figure out really fundamental things about how the world works, including light, motion, and gravity.

One really influential Greek scientist, Aristotle , had famously argued that heavier objects fall faster, so a feather and a stone fall at different speeds because the stone weighs more.

In perhaps his most famous experiment, Galileo proved this was false. Apparently he dropped balls weighing different amounts from the Leaning Tower of Pisa in Italy.

Despite their different weights, the two balls reached the ground at exactly the same moment—proving Aristotle was wrong. According to Galileo, a feather will hit the ground more slowly than a stone because air resistance slows the feather down as it falls.

Interestingly, when scientists from the Apollo 15 mission went to the moon where there is no air resistance to slow feathers down , they carried out the feather and stone experiment with a very satisfying result, as you can see in this video clip from Wikipedia.

Everyone loves a rainbow, but where do those amazing colors come from? The classic experiment that showed how ordinary light is made up of different colored light was carried out by English scientist Isaac Newton, unquestionably one of the greatest scientists who ever lived.

He trained sunlight from his window onto a triangular-shaped wedge of glass a prism and split it into dazzling colors. Quite an undertaking, you might think! His apparatus was relatively simple. He had two small balls mounted on the ends of a stick and two larger ones mounted on a second stick.

The larger balls would swivel back and forth, attracted by the gravitational force that the smaller balls exerted on them.

So the Cavendish experiment laid the foundations for our modern theories of gravity. In , Thomas Young dreamed up a classic experiment.

He made two narrow slits in a board and placed a light beam between them so it shone through both slits simultaneously onto on a wall behind. If Newton had been correct about light, Young would have seen a central bright area on the wall and darkness either side. But when he did this experiment, what he actually saw was a pattern of light and dark areas where the light rays from the two slits "interfered.

This interference pattern proved that the light rays were traveling not as particles but as waves. Left: A laser 1 produces coherent regular, in-step light 2 that passes through a pair of slits 3 onto a screen 4. What we actually see is shown on the right. Light appears to ripple out in waves from the two slits 5 , producing a distinctive interference pattern of light and dark areas 6.

In , Albert Einstein proved that light could indeed behave like a particle: if you shone light on a metal, you could knock electrons out of it to make an electric current a phenomenon known as the photoelectric effect and the science for which Einstein won the Nobel Prize in Physics. As a result of this, people came to realize that light was a particle and a wave—an idea now known as wave-particle duality, which is one of the key ideas in quantum theory the area of physics concerned with atoms and other atomic-scale phenomena.

Remarkably, he saw the same interference pattern, proving that electrons could be considered as waves as well as particles. Want to know more? Science teacher Derek Owens has made an excellent little animation explaining the double-slit experiment that you might like to watch. Suppose you want to run a marathon. A basic law of science called the conservation of energy tells us you need to fill your body with "42 km 26 miles worth of food.

In other words, anything you want to do needs energy to do it. The person who figured this out experimentally was James Prescott Joule. In his experiment, there was a large container full of water that had a paddle wheel fixed inside it.

The paddle wheel was connected to an axle around which a string was wrapped many times. The string was looped over a pulley and had a heavy on the end of it. When Joule released the weight 1 , it pulled the string around the pulley 2 , turned the axle 3 , and made the paddle wheel spin, which heated up the water. He let the weight fall about 20 times so the water heated up enough for him to be able to measure.

How can we measure its speed at all? Artwork: How Fizeau measured the speed of light. Then, in the middle of the 19th century, French physicist Armand Hippolyte Louis Fizeau figured out a way to measure the speed of light on Earth. He shone a beam of light 1 at a half-silvered mirror 2 so it bounced through a wheel rotating hundreds of times per second 3.

Like a gear , the wheel had teeth cut into its edge and the light shot through one of them. Fizeau arranged for a mirror 4 to be positioned about 8. He knew how far the light beam had traveled, so all he had to measure was how long it took.

The rotating gear wheel was effectively his clock: knowing how many teeth it had and how fast it was spinning, he could adjust its speed until it just blocked out the light from the far mirror. At that point, he knew that the light beam had traveled only once from his lamp to the mirror and back again a distance he had measured , and he also knew how much time had elapsed between the light beam departing and coming back again.

So all he had to do was divide the distance by the time to calculate the speed of light. His figure was about 3. This more accurate technique enabled him to measure the speed of light as 2. The School Physics website has a great page about measuring the speed of light, showing how Fizeau and Foucault made their measurements and the calculations they used, and how even more accurate results were later obtained by American physicist Albert Michelson.

How can you possibly measure the charge on something so small? Robert Millikan figured out a way to measure the smallest unit of electric charge by spraying oil droplets between two electrically charged plates that were suspended horizontally. After giving them an electric charge, he found he could move them up and down by adjusting the voltage on the plates, and by measuring the speed of their motion he could calculate the charge that they had.

He then gave the droplets a negative electric charge so he could stop them falling by applying a positive voltage to the upper plate. In other words, so their weight acting downward was balanced by an attractive electrical force acting upward.

With the power switched on, he found that some drops fell more slowly, some stopped moving, and some even moved upward. A bit of clear thinking told him that drops must be carrying multiples of the basic unit of electric charge multiple electrons, in other words and this affected how quickly they rose or fell when the power was on. By measuring their terminal velocity with the power on, and comparing it to their terminal velocity with the power off, he calculated the basic unit of electrical charge—now known as the charge on the electron—with reasonably high accuracy.

This important work won him the Nobel Prize in Physics. The phrase "splitting the atom" means different things to different people. Perhaps the fairest way of looking at it is to say that it refers to a whole series of experiments that took place from about to about , when a group of brilliant scientists identified the parts inside atoms and worked out how they were arranged.

The atom-splitting experiments included J. Artwork: Transmutation: When Rutherford fired alpha particles helium nuclei at nitrogen, he produced oxygen. As he later wrote: "We must conclude that the nitrogen atom is disintegrated under the intense forces developed in a close collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus.

While working at Manchester University in England, Rutherford got two of his students, Hans Geiger and Ernest Marsden, to fire positively charged alpha particles at a thin sheet of gold foil. As expected, most of the particles shot straight through but a tiny number roughly one in were bent through large angles and some even bounced right back. Rutherford and his colleagues were astonished. As he famously stated: "It was almost as incredible as if you fired a inch shell at a piece of tissue paper and it came back and hit you.

Most of the alpha particles shot straight through this electron cloud and were unaffected. The few that were deflected had been fired very near or directly at the nucleus, so their positive charge was repelled by the positive charge there. It was this experiment that confirmed our modern picture of the atom with a central nucleus and electrons arranged around it. This is sometimes called the Rutherford atom , which was later improved by Niels Bohr to produce the Rutherford-Bohr model of the atom.

Any particles fired at the nucleus are deflected by its positive charge 3. Fired at exactly the right angle, they will bounce right back! While this experiment is not splitting any atoms, as such, it was a key part of the decades-long effort to understand what atoms are made of—and in that sense, it did help physicists to "split" venture inside the atom. By this point, scientists had figured out the structure of the atom. Putting these two things together, it followed that you ought to be able to smash atoms apart and release huge amounts of energy.

Artwork: The nuclear chain reaction that turns uranium into uranium with a huge release of energy. Fermi tested this out at the University of Chicago with an experimental setup he called an "atomic pile.

Uranium has one more mass unit than uranium, thanks to the added neutron, but it is so unstable that it immediately splits up into two smaller atoms 3 and two neutrons 4. The two neutrons then flew off and hit two other uranium atoms, making two more reactions happen This is the famous chain reaction that powers nuclear bombs and nuclear power plants.

Photographed with X rays, these intertwined curves appear as an X shape. X ray diffraction works a bit like shadow-play puppetry, only instead of using a flashlight to cast shadows of your hands and make animal silhouettes on a wall, you use an X ray beam to throw precise shadows of the atomic structure of a material onto a photographic plate. The photo you get can reveal how atoms are arranged inside a crystal and the spacing between them.

Crick, Watson, and Wilkins were rightly celebrated for their discovery, but one key member of the team was missing from the Nobel roll call: Rosalind Franklin, who had died of cancer four years earlier in , aged just 37 Nobel Prizes are never awarded posthumously.

Franklin had taken a particularly important X ray diffraction photo, which revealed a huge amount of information about the structure of DNA. Sponsored links.


The 10 greatest physics experiments?





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