light

Electromagnetic waves (made up of electric and magnetic components) in the visible range, having a wavelength from about 400 nanometres in the extreme violet to about 700 nanometres in the extreme red. Light is considered to exhibit particle and wave properties, and the fundamental particle, or quantum, of light is called the photon. A light wave comprises two transverse waves of electric and magnetic fields travelling at right angles to each other, and as such is a form of electromagnetic radiation. The speed of light (and of all electromagnetic radiation) in a vacuum is approximately 300,000 km/186,000 mi per second, and is a universal constant denoted by c.

Light is a form of energy that is mainly visible to the human eye. Light is radiated by hot objects such as the Sun or an electric light bulb.

The nature of light travelling through a medium can be explained with the help of Young's double-slit experiment. A double slit is placed between a light source and a screen. Light diffracted by the double slit forms a pattern of bright and dark bands (called fringes) on the screen. The fringe pattern is due to the light waves moving through the slits and being diffracted. At a point where light waves are in phase (a crest and crest meeting at the same point) a constructive wave front is formed with a larger amplitude than that of the two waves forming the constructive wave front. This is seen as a bright band on the screen. At a point where the light waves are out of phase (a crest and a trough meeting at the same point) a destructive wave front is formed with zero amplitude (the waves effectively cancel each other out). This is seen as a dark band on the screen.

Light from the Sun The Sun produces light and this is vital to life on Earth. Apart from providing the light for us to see by, it provides energy for nearly all organisms on Earth. Many food chains begin with a green plant that makes high-energy food by photosynthesis using the energy of absorbed sunlight. This process occurs mainly in the leaves of a plant – through the action of chloroplasts in the cells of the leaf. Herbivores eat the plants to obtain some of this food and carnivores eat the herbivores for the same reason. Other food chains may start with the dead remains of plants or animals. However, in all cases high-energy food is passed along these food chains and the energy has all been obtained from the Sun.

Plants show responses to light. Generally, stems grow towards the light. This growth is controlled by auxins.

Although much light is visible to humans, there are some types of light that are not. Ultraviolet light (UV light) is an example. UV light damages DNA and causes mutation. The Sun produces UV light, although much is absorbed by the atmosphere before it reaches the ground. However, exposure to bright light increases the risks of skin cancer in humans.

Early discoveries Isaac Newton was the first to discover, in 1666, that sunlight is composed of a mixture of light of different colours in certain proportions and that it could be separated into its components by dispersion. Before his time it was supposed that dispersion of light produced colour instead of separating already existing colours. The ancients believed that light travelled at infinite speed; its finite speed was first discovered by Danish astronomer Ole Römer in 1676.

Optics is a very ancient branch of physics. It is stated that a lens of rock crystal was found in the ruins of Nineveh, while Aristophanes mentions the use of burning-glasses in The Clouds. Reflection and refraction were known to the Greeks in 300 BC, and theories of vision were formulated by the Pythagoreans and the Platonists. Cleomedes, a Roman of the time of the Emperor Augustus, following Ptolemy, extended the knowledge of refraction and explained that atmospheric refraction enables us to see the Sun after it has set. Alhazen (about 10th century AD) wrote a book on optics, and, in addition to advancing the knowledge of reflection and refraction, made a close study of the optics of the human eye. Roger Bacon (13th century) made notable contributions and prophecies in optics, some of which bore fruit when Galileo constructed one of the first telescopes in 1609. Willebrord Snel of Leiden discovered the law of refraction about this time, and Newton explained it by the assumption of a corpuscular theory of light. According to this theory a luminous body emits swarms of corpuscles that travel in straight lines through space. Christiaan Huygens, a contemporary of Newton, formulated a wave theory of light, but Newton's great contributions to the knowledge of light, combined with his great reputation, caused his theory to be favoured, and it was left to Augustine Fresnel (1788–1827) and Thomas Young (1773–1829) to establish Huygens's theory by the evidence of their experiments on diffraction and interference.

Modern theories The 19th century saw the

development of the theory of the ether, the medium through which light was supposed to be propagated, but this finally gave place to the electromagnetic wave theory following James Clerk Maxwell's theoretical researches supported by the accurate determinations of the velocity of light. The emission of light from self-luminous bodies is an atomic phenomenon that was given a satisfactory explanation by Max Planck, Niels Bohr, and others, in terms of the quantum theory. Similarly the absorption of light, and in particular the emission of electrons from metallic surfaces illuminated by light (photoelectric effect), was explained by the quantum theory. Certain properties of light, however, are explained only by the hypothesis that light is propagated as electromagnetic waves. Thus the quantum theory accounts for the photoelectric effect, while the electromagnetic wave theory accounts for the interference of light. The relationship between these two theories can be approached in terms of Werner Heisenberg's uncertainty principle.

Scientists have succeeded in stopping a pulse of light, keeping it ‘parked’ for a time, and then restarting it. One technique involved shooting a short pulse of red laser light into a gas of rubidium atoms, and using two control lasers to interact with these atoms. This interaction created structured layers that alternately reflected and transmitted the light so that it effectively stopped in place.

See also refraction; diffraction; interference.