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Light

A close-up of a halogen light bulb, showing the filament, a thin wire wound in a tight coil, made of tungsten. When electricity passes through the coil it becomes white hot. The bulb contains a small amount of a halogen, such as iodine or bromine. The combination of the halogen gas and the tungsten filament produces a chemical reaction in which evaporated tungsten recoats the filament, prolonging its life.Close-up of a halogen light bulbA close-up of a halogen light bulb, showing the filament, a thin wire wound in a tight coil, made of tungsten. When electricity passes through the coil it becomes white hot. The bulb contains a small amount of a halogen, such as iodine or bromine. The combination of the halogen gas and the tungsten filament produces a chemical reaction in which evaporated tungsten recoats the filament, prolonging its life. Light is the form of energy that our eyes can detect, enabling us to see. It is produced by very hot things—stars like the Sun, fire and the tiny wires inside electric light bulbs. Certain animals also have light-producing organs. Light rays can travel only in straight lines, although they can be reflected. If they strike an object which does not allow light to pass through (an opaque object), a shadow is cast on the unlit side. Sunlight is made up of all the colours of the rainbow: the spectrum of light. Light from the Sun is essential to most life-forms on Earth. Plants use sunlight to make their food, a process called photosynthesis. Thus all plant-eating animals, together with other animals that eat plant-eaters, also depend on sunlight.

An illustration showing different sources of light: moonlight (reflected sunlight), glow worms in a bush and light produced by electricity: torches, car headlights and a ship's lights.An illustration showing different sources of lightAn illustration showing different sources of light: moonlight (reflected sunlight), glow worms in a bush and light produced by electricity: torches, car headlights and a ship's lights.
Stars are so distant from Earth that their light may take many years to reach us, even travelling at 300,000 kilometres per second.Starlight takes many years to reach usStars are so distant from Earth that their light may take many years to reach us, even travelling at 300,000 kilometres per second.

The speed of light

When we switch on an electric light, it seems that the room is filled with light instantaneously. But light rays do take time to travel from their source. They travel extremely quickly: about 300,000 kilometres (186,000 miles, or seven-and-a-half times around the world) per second in outer space. The speed of light is, in fact, the speed limit for the Universe: nothing can travel faster. Light waves can travel through empty space as well as through air, water or glass—although they travel more slowly through these. Because stars are very far from Earth—at least thousands of billions of kilometres—it is easier to give their distances in light years: the amount of time it takes for light to travel to us from them.

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Light

The speed of light

It takes about 8 minutes 17 seconds for the Sun's light to reach the Earth.