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Light is so common that we often take it for granted.

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Light is so common that we often take it for granted.  Yet the world would quickly change if suddenly there were no light.  We could no longer see, because light that comes to our eyes makes seeing possible.  Without light, we would have no food to eat or air to breathe.  Green plants use the light from the sun to grow and to make food.  All the food we eat comes from plants or animals that eat plants.  As plants grow, they give off oxygen.  This oxygen is a necessary part of the air we breathe.  

Light gives us fuels.  The energy in the sunlight that shone on the earth millions of years ago was stored by plants.  After these plants died, they were changed into coal, natural gas, and oil.  Today, we use the energy in these fuels to produce electricity and to operate machines.  

Light from the sun also heats the earth.  Without the sun's light, the earth would soon become so cold that nothing could live on it.  Even if we burned all our fuels, we could not keep the earth warm enough for life to exist.  People have found ways of making and controlling light in order to see when there is no sunlight.  At first, they produced light with campfires and flaming torches.  Later, they developed candles, oil lamps, gaslights, and electric lights.  

People make and use light for many other purposes than to see by.  For example, the pictures on a television screen consist of spots of light.  Using scientific instruments, people can study light itself and learn much about the universe.  For example, the light from distant stars can tell scientists what the stars are made of.

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The visible spectrum forms only a small part of the full range of electromagnetic waves.  Waves that have wavelengths slightly too short to be seen are called ultraviolet rays.  They cause suntan, sunburn, and skin cancer.  Waves with somewhat shorter wavelengths than ultraviolet rays are called X rays.  These rays can penetrate a person's body.  Doctors and dentists use them to "see" inside the body.  Gamma rays have even shorter wavelengths than X rays.  They result from nuclear reactions, such as those in the sun.  

Waves with wavelengths slightly longer than those of red light are called infrared rays.  When you stand in bright sunlight or in front of a fire, you feel warm largely because of the infrared light shining on you.  Microwaves and radio waves have longer wavelengths than infrared waves.  A microwave oven shines microwaves on food to warm it.  A police officer's radar unit shines microwaves onto an approaching car to measure its speed.  Radio and television stations broadcast programmes by sending radio waves.  

Sunlight spread into its different colours by a prism creates a continuous spectrum.  From violet to red, the spectrum blends smoothly from one colour to the next.  Many other sources of light do not produce a continuous spectrum.  For example, a street lamp may produce bright yellow, blue, and a few dimmer colours, but it also has dark regions in its spectrum.  The colours are produced by certain atoms in the gas inside the lamp.  For example, the yellow comes from sodium atoms.  Each type of atom can produce only certain colours.  

Scientists can learn what kinds of atoms make up a light source by observing what colours are present in the light.  They direct the light through an instrument called a spectrometer to separate the colours.

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All attempts to observe or measure the properties of the ether failed.  Scientists became increasingly convinced that the ether did not exist.  Experiments conducted by Albert Michelson and the American physicist Edward Morley in 1887 helped destroy the ether theory.  

Quantum mechanics.  In 1900, the German physicist Max Planck discovered an equation that matched experimental data about the emission of light by a hot surface.  Planck could not explain why the equation worked.  But he realized that it predicted that the tiny emitters of light on the surface can have only certain values of energy.  When energy is restricted to certain values, it is said to be quantized.  

In 1905, Einstein revealed that light itself is quantized.  Einstein reasoned that if light emitters can have only certain values of energy, then the energy they emit as light will retain its quantized character.  The light comes in tiny packets of energy that are known as quanta.  The concept of light as quantized energy explained how light behaves as a particle in certain experiments, instead of as a wave.  These particles of light came to be called photons.  

In 1913, the Danish physicist Niels Bohr proposed that the energy of atoms was also quantized.  When energy is given to an atom, either by a collision or by shining light on it, the atom can accept only certain values of energy.  In this way, the atom becomes excited.  When it de-excites, it must get rid of the extra energy.  One way it can do this is by emitting a photon that carries the energy away.  Each type of atom accepts a different set of energies.  Thus, when atoms emit light, the photons from one type of atom differ in energy from the photons from other types of atoms.  

A field of physics known as quantum mechanics is the study of how atoms and light are quantized.  Quantum mechanics involves the fact that light and matter behave as waves in some experiments and as particles in other experiments.

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