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.  It can also tell them if the stars are moving toward or away from the earth and how fast they are moving.  See RED SHIFT.  

Scientists also use light to identify and study chemical substances.  In addition, light is used for communication.  Optical cables transmit information in the form of light over long distances.  They are increasingly replacing the copper wires used by telephone companies.  Scientists are now experimenting with light as a carrier of information inside very fast central processors of computers.  See FIBRE OPTICS.  

What is light?  This question has been a puzzle for centuries.  People once thought light was something that travelled from a person's eyes to an object and then back again.  If anything blocked the rays from the eyes, the object could not be seen.  They argued also that light needed a medium, which they called the ether, to travel through space.  Since the 1600's, scientists have made many discoveries about light.  They have learned that light is a form of energy that can travel freely through space.  The energy of light is called radiant energy.  There are many kinds of radiant energy, including infrared rays, radio waves, ultraviolet rays, and X rays.  We can see only a tiny part of all the different kinds of radiant energy.  This part is called visible light or simply light.  

This article describes where light comes from, the nature of light, and how light behaves when it comes in contact with various materials.  The article also tells how light is measured and discusses the important scientific discoveries about light.

                    LIGHT/Sources of light

Light makes it possible for us to see.  Many of the things we see, such as the sun and room lights, are sources of light.  We see all other things because light from a source bounces off them and travels to us.  Light sources can be classified as natural or artificial.  Natural light comes from sources that we do not control.  Such sources include the sun and the stars.  Artificial light comes from sources that we control.  These sources include candles and car lights.  

How light is produced.  All light comes from atoms.  It is produced by atoms that have gained energy either by absorbing light from another source or by being struck by other particles.  An atom with such extra energy is said to be excited.  Ordinarily, an atom stays excited only briefly.  It de-excites by giving up its extra energy.  It can either run into another atom to lose the energy, or it can emit (give off) light.  The light then carries away the extra energy.  The amount of energy needed to excite atoms and the amount of energy the atoms emit as light varies for different kinds of atoms.  

Light is usually described as a wave, shaped much like a water wave that moves across a lake.  But light can also be described as a small particle, called a photon.  Each photon moves in a straight line, much as a pool ball does.  In both descriptions, the light has energy.  The amount of energy that is carried by the wave or photon largely determines the colour of the light.  For example, suppose you see a red apple on a blue chair.  Each photon from the apple has less energy than a photon from the chair.  

One way to excite atoms so that they emit light is by heating them.  A poker may be heated until it is white-hot.  Because of the heating, the atoms at the poker's surface collide violently with each other.  When they collide, they excite one another.  Each atom quickly emits its extra energy as light but is almost immediately re-excited by another collision.  These collisions produce such a variety of states among the atoms that the photons released have a wide range of energies.  The combination of all the resulting colours is white light.  As the poker cools, fewer atoms are excited to high energies, and so the atoms emit fewer photons with the higher energies of blue light.  Since red light is still being emitted, the cooling poker looks red.  

Other sources of light.  Many substances gain energy and emit light without being heated very much.  They do this through a process called luminescence.  Some luminescent materials glow in the dark long after they have received extra energy.  They are said to be phosphorescent.  Their atoms stay excited for some time before they de-excite and emit light.  Certain phosphorescent materials are used in the markings that glow on watch faces (see PHOSPHORESCENCE).  Other luminescent materials emit light only during their exposure to exciting energy.  They are said to be fluorescent (see FLUORESCENCE; FLUORESCENT LAMP).  

Fireflies and a few other types of organisms emit light by a process called bioluminescence.  In this process, chemicals within the organisms combine to produce a different chemical that has excited atoms.  When the atoms de-excite, they emit photons.  

The sun shines because nuclear reactions between hydrogen atoms within its core produce a tremendous amount of energy.  Photons and other kinds of particles carry the energy to the sun's surface.  At the surface, these particles excite atoms that then de-excite by emitting light.  The earth receives part of that light.  All stars emit light by this process.  

An aurora such as the northern lights is an emission of light by molecules of air.  When high-speed particles arrive at the earth from large eruptions on the sun, they crash into the air molecules.  These collisions excite the molecules with extra energy.  The molecules then release the energy by giving off light.  When the collisions occur at night, the light emitted may be bright enough to be seen.  

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A laser is a device that produces a powerful, narrow beam of light in which all the photons have the same energy and travel in the same direction.  Lasers serve as tools in scientific research, surgery, and telephone communications.  They also have many industrial and military uses.

                    LIGHT/The nature of light

During most of the 1800's, scientists thought of light as a wave that travels much like a water wave.  This idea of light as a wave was popular because it explained experiments in which ...

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