How is light produced? Stopping light Sources of light Speed of light

Light is a form of energy that the eye can detect. As you read this on your computer screen, light energy is being transmitted to your eyes, and your brain is interpreting the images.

Light is similar to sound in that it can be considered to travel as waves with measurable wave lengths. There are, however, some major differences. One of these is that light travels much faster than sound. Secondly, the wave length of light determines its colour, whereas with sound, the wave length determines the pitch of the sound. Another important difference is that light does not need a medium to travel through, like air or any other matter, because it does not depend on vibration of particles in a medium to exist. Light is considered to exist as little bundles of energy called photons. Photons have both wave and particle-like characteristics and our understanding of their behaviour is still evolving.

How is light produced?
There are a few different ways in which light is produced, but first you need to know a little about matter itself. Matter is made of atoms, and each atom has a nucleus containing protons and neutrons, with electrons orbiting the nucleus. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge at all. See the topics Atoms and Atomic theory - modern models.

One of the most important sources of light is vibrating atoms in hot objects. When a substance is heated, these vibrating particles give off energy, in the form of electromagnetic radiation; visible light is part of this electromagnetic radiation. The hotter the object, the shorter the wavelength of the radiation will be. Its colour will change from red to yellow and so on. The spectrum of radiation depends only on the temperature of the object, not what it's made of. Very hot objects emit all wavelengths of visible light and appear to glow white. Because of this, it is called thermal radiation. Even your body emits thermal radiation, but the energy is in the infrared region. You can't see this radiation, but if your eyes were able to detect it, people would glow! (Some cameras can actually detect infrared radiation and allow you to "see" invisible objects).

Another way in which light can be produced is by electrons jumping from one energy level to another within an atom. Electrons can only have certain exact amounts of energy called quanta, if they drop to a lower energy orbit they will give off light of an exact wavelength. Examples of this are the aurora in the atmosphere, which glows with very specific colours and neon lights.

Light from an atom

Stopping light
Unlike sound, which can pass through most materials as vibrations, light is absorbed by most materials. Light energy is easily absorbed by most things producing sharp shadows, demonstrating that light travels in straight lines. Objects that absorb light are said to be opaque.

Objects that let a large percentage of light through are said to be transparent. We can peer through them and still see objects on the other side, examples are clear glass and crystal. Other objects that let light through, but cause the light to be diffused producing unclear "images" of objects behind them, like ice and frosted glass, are said to be translucent. Light can also change direction, when reflected from mirrors, for example. Look around you to see if you can spot transparent, translucent, opaque and reflective objects.

Sources of light
Apart from the Sun, which is the dominant light source for our Earth, light can be produced by objects converting their potential energy to light energy. Candles convert chemical energy stored in the candle wax to light, logs burn, glow worms and deep sea fish can use chemical potential energy stored in phosphorus compounds to produce their own light.

Another spectacular light source is the lightning that can be observed during a thunderstorm. Your home, car, street, even the indicator lights on your computer all produce energy that your eyes can see, called light. In each case other forms of energy have been transformed into light.

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