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Light waves and particles
Scientists have attempted to answer the question "what is light?" for hundreds of years. The English scientist Isaac Newton (1642–1727) believed light was made up of microscopic balls or particles. The Dutch scientist Christiaan Huygens (1629–95) proposed that light travelled as waves, rather like sound or ripples on the surface of water. But when it came to explaining how light is absorbed and emitted by atoms, light has to be described not as waves but as packets of energy, or particles called photons.
Waves
A light wave travels in one direction, wobbling up and down as it does so. It has crests (or "peaks") and troughs, a bit like ripples on a pond. As it moves, there is always the same distance between crests and troughs; this distance is called the wavelength. The height of a crest or the depth of a trough is known as the wave's amplitude. The greater the amplitude, the greater the wave's energy. If you could count the number of wave crests that went by in one second, that number would be the frequency of the wave: this is always the same for that wave. The unit of frequency, a measure used to compare frequencies of all waves, including sound, radio and light waves, is the hertz (Hz), named after the German physicist Heinrich Hertz (1857–94).
Waves move at different speeds through different materials. The greatest speed of a light wave is in a vacuum, such as in outer space: equal to about 300,000 kilometres per second (186,000 miles per second). It is slightly slower through air, since there are air molecules to slow it down. It is slower still through transparent liquids and solids.
Each colour light wave has its own wavelength: red light has nearly twice the wavelength of violet light. Yellow, green and blue light have wavelengths in between the two.
Light as waves
The English scientist Thomas Young (1773–1829) "proved" the wave theory of light by demonstrating that when two light rays crossed, a pattern of bright and dark areas appeared. This could be explained by the fact that the crests and troughs of one wave mingled with those of the other. Young's work, and that of French physicist Augustin Fresnel (1775–1812), demonstrated that wave theory could explain many of light's properties, showing how it can undergo reflection, refraction, interference and diffraction.
The wave theory of light also explains another property of light: its colour. As light waves pass through a prism, they are bent or refracted. Longer waves of red light refract least, while shorter waves of violet light refract most. The other colours, orange, yellow, green and blue, are spread out between them.
Electromagnetic waves
Drawing on observations about electricity and magnetism, the Scottish physicist James Clerk Maxwell (1831–79) predicted the existence of electromagnetic waves. Light is just one form of electromagnetic wave, with its own particular range of wavelengths. Other forms of energy have longer wavelengths (infrared and radio, for example) or shorter wavelengths (ultraviolet and X-rays), and thus lower or higher frequencies. All can travel through a vacuum, and all do so at the same speed, the speed of light.
Diffraction, interference and superposition
When a beam of light passes through a narrow slit it spreads out. The narrower the slit, the wider the spread. This is called diffraction. If two diffracted beams overlap, the wave nature of light causes an effect known as interference.
This can be demonstrated by passing a narrow beam of light through two slits on to a screen beyond (the "double-slit experiment"). The resulting two diffracted beams "interfere" to produce a pattern of light and dark stripes on the screen.
Where the crests of both waves reach the screen at the same time, the combined intensities produce a bright line. This is known as superposition, or constructive interference. The same thing happens when two troughs coincide. Where a crest of one wave arrives at the same time as the trough of another, they cancel each other out, leaving a dark line. This is called destructive interference.
The patterns of colour that appear on soap bubbles are caused by light interference. Light rays reflected from the inside of the thin film of soap travel slightly further to the eye than those reflected from the outside. White light is made of waves of many different wavelengths, and, after they have been reflected, some of those waves interfere constructively, so their colour becomes brighter. Waves of other wavelengths interfere destructively, so their colours are dimmed.
Scattering
When light passes through the air it strikes the minute particles of air (gas molecules) and dust and is scattered (sent in many directions). When sunlight passes through the atmosphere, some of its waves are scattered. Sunlight is made up of all the colours in the spectrum. Different colours of light have different wavelengths. Blue waves of light travel in shorter waves than other colours. Shorter wavelengths of light are scattered by gas molecules more than other colours—and this is why the sky usually looks blue; the blue rays have been scattered all over the sky.
When the sun is low in the sky at sunrise or sunset, the blue light must pass through more air and some of it is scattered away in other directions. We see less of the blue light and more of the unscattered red light.
Polarization
Ordinary (unpolarized) light waves vibrate in all directions at right angles (transverse) to the direction they are travelling. A polarizing filter transmits only light that is vibrating in one direction: it is polarized. All other light is absorbed. This effect is used in polarizing sunglasses, which help to reduce glare.
Transparent, translucent or opaque
When light falls on an object it can be either transmitted, reflected or absorbed. If the object is transparent, such as water or glass, most of the light is transmitted—it passes directly through it. A translucent material, such as some glasses, plastics or liquids, transmits light, but the the light is scattered by tiny particles inside the material, giving it a milky appearance. If the object is opaque, some may be reflected and some absorbed, depending on the type of surface. White or shiny surfaces reflect light, while dark surfaces absorb it.
Absorption
The electrons in atoms, the building blocks that make up all matter, can absorb light, but only that of a few certain colours; the actual colours depends on the kinds of atoms. When white light, which is made up of many different colours, falls on a material, the atoms that make up that material absorb some of the colours, while reflecting others. The reflected colours, as seen by the eye, give the material its colour.
Light as particles
Wave theory offers no complete explanation as to how light can be absorbed and re-emitted by atoms. Instead, light has be to be described not as waves but as particles: minuscule packets of energy. These particles, called photons, interact individually with electrons in atoms.
Quantum theory
In 1905 Albert Einstein (1879–1955), following earlier work by the German physicist Max Planck (1858–1947), proposed that light was neither purely a wave nor purely a particle, but a combination of both. Light (along with other forms of radiation) consisted of units of energy, each called a quantum of energy (from the Latin word quantus meaning "how much"). A quantum of light energy is called a photon. Light was a stream of such particles, with the energy content dependent on its frequency. A "blue" photon—at the higher frequency end of the spectrum—had more energy than a "red" one. This is called quantum theory.
When an electron in an atom loses energy, it does so by releasing a photon. And, when an electron absorbs a photon, its energy increases. A bright red light is made up of many photons, but each has a low energy. A dim blue light has few photons, but each has a high energy. White light is a mixture of photons of different energies.
Generating electricity with light
When light shines on to a metal, it can excite and dislodge electrons from the metal atoms and so generate electricity. This is called the photoelectric effect. Increasing the intensity of the light increases the number of electrons released, but does not produce electrons of greater energy.
In the photovoltaic effect, light gives up some of its energy to the electrons in a material. The electrons are then free to move through the material, creating an electric current. A device which converts the light energy from the sun into electrical energy is called a solar cell. Solar panels, used for generating electricity, are made up of a number of solar cells.
Fluorescence
When an atom absorbs visible light or ultraviolet (UV) it becomes excited and emits (gives off) energy. It does so by re-emitting visible light or UV. When a chemical absorbs UV and emits energy in the form of visible light, this is called fluorescence. The effect is used in "glowing" dyes, high-visibility clothing, signage, lamps and scientific research. Fluorescence occurs naturally in some minerals and a number of animals.
Fluorescent materials cease to glow almost as soon as the radiation source stops. Materials that continue to emit light for some time are known as phosphorescent.
Spectroscopy
The study of the absorption and emission of light by atoms is called spectroscopy. It gives scientists a way of analyzing and identifying materials and is useful in scientific research, medicine, astronomy and industry. Using an instrument called a spectroscope, light emitted from an object or material is recorded as a spectrum. Black areas on it correspond to gaps where the wavelengths of certain colours have been absorbed. Each element has a different absorption spectrum, its own "fingerprint".
Consultant: Mike Goldsmith