The wave motion of lightIn those early days no one knew exactly what vibrated in light to make it behave as a wave motion, nor how the waves could be produced. These problems began to be solved in the 1870s. It was discovered that a light wave consists of vibrating electric and magnetic fields traveling through space; the two fields vibrate at right-angles to each other and to the direction of motion. In fact light waves are part of a whole group of electromagnetic waves that include X-rays, ultraviolet rays, infrared rays, and radio waves. Light waves can be produced by changing the orbits of electrons inside atoms. If an atom receives energy in some way – perhaps as heat, light or electrical energy, for example – the electrons move away from the nucleus to orbits of higher energy. They then jump to a lower energy orbit and give out energy if the form of electromagnetic waves as they do so. In this way objects produce light.
Light spreads out from any point producing it or reflecting it in ever-expanding spheres, rather as ripples spread out in circles over a pond. Each ray of light can be thought of as moving in a straight line, producing a continuous series of ever-expanding vibrational movements through space. In all the rays leaving a point the vibrations add up to give a set of spherical wavefronts consisting of alternate peaks and troughs of energy. Each peak and trough are maxima of vibration but in opposite directions.
The shadow of an object is rarely seen to have sharp edges, but this is because a source of light always has a certain size. If the source were infinitely small we would expect it to give shadows that are totally sharp because light rays are considered as straight lines, but this is not so. All waves spread round the edges of an object – an effect called diffraction. In the case of light the edge is illuminated and points close to it can act as sources of light waves that spread out in all directions so that the rays are effectively bent by the edge. The wavelength of light is so short that this effect is hard to detect at edges, but it becomes clearly apparent when light passes through very small openings about the same size as the wavelength. This happens in a diffraction grating in which light passes through or is reflected from extremely narrow slits.
The effects of interference
If a ray of light is divided into two rays that later recombine then interference effects are seen if one of the divided rays travels a longer path than the other before the recombination. The peaks and troughs may be out of phase (not exactly together) and the light is affected. This happens between two surfaces that are very close together, as in a thin film or two pieces of glass pressed together, and it produces colorful fringed patterns. The iridescent colors seen in the plumage of some birds and some butterflies' wings are produced by the phenomenon of interference; the fine structure of the feather or wing resembles either a diffraction grating or a thin film.
Because interference can be produced by a path difference of only a wavelength or so, interference effects can be used to detect very small changes in length. Interferometers are used for this purpose. They produce interference by dividing a ray of light into two or more beams and then recombining them.
Polarization of light wavesAnother effect to be seen with light waves is polarization. In an ordinary light wave the electric and magnetic fields vibrate in many randomly oriented planes about the direction of wave motion; in polarized light they vibrate in only one plane. Light is polarized by passing it through a filter that cuts out all vibrations except those in one particular plane. The polarized beam will then pass through a second filter only if it is set at the correct angle to allow the vibrations through. Otherwise the beam is stopped. Light reflected from surfaces at certain angles is polarized, and polarizing sunglasses cut out glare by stopping reflected beams in this way. Solutions of some chemical substances, such as various sugars, rotate the plane of polarization of light passing through them. The effect is used in chemistry for analyzing such solutions.
Related category OPTICS AND OPTICAL PHENOMENA
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