THE NATURE OF LIGHT
Until about the middle of the I7th century, it was generally believed that light consisted of a stream of corpuscles. These corpuscles were emitted by light sources, such as the sun or a candle flame, and travelled outward from the source in straight lines. They could penetrate transparent materials and were reflected from the surfaces of opaque materials. When the corpuscles entered the eye, the sense of light was stimulated. This theory was called on to explain why light appeared to travel in straight lines, why it was reflected from a smooth surface such as a mirror with the angle of reflection equal to the angle of incidence, and why and how it was refracted at a boundary surface such as that between air and water or air and glass.
For all of these phenomena, a corpuscular theory provides a simple explanation.
Newton (1687) explained the facts of refraction on the basis of the corpuscular theory (in which light was supposed to consist of small particles which in their motions along straight lines finally struck the eye and produced the sensation of light). He assumed that the particles moved with greater speed in water than in air, an assumption which could neither be proved nor disproved at the time.
Newton's experiment, with the prism, however, constituted a great forward stride. It had been believed that white light was changed in its fundamental nature by passage through a prism. Earlier writers had associated the colours of the rainbow in some way with refraction, but it was not until Newton's experiment that anything definite was known.
Newton had proved that white light is a mixture of colours and that rays of light of the various colours are bent by varying amounts when passing obliguely from one medium to another of different density, each colour having a different index of refraction.
Newton's discoveries relating to colour and refraction were later to have an important bearing on the wave theory of light. We know colour to be determined by the length of the
light waves, red light having a longer wave length than blue light. Newton explained everything on, the basis of his corpuscular theory, which he had invented to satisfy the requirements that light must travel in straight lines.
By the middle of the I7th century, while most workers in the field of optics accepted the corpuscular theory, the idea had begun to develop that light might be a wave motion of some sort.
In I670 Huygens showed that the laws of reflection and refraction could be explained on the basis of a wave theory and that such a theory furnished a simple explanation of the recently discovered phenomenon of double refraction.
Lomonosov studied the phenomena of light and believed it to be a wave motion. In 1753 Lomonosov informed the Academy of Science of his intention to make experiments with strings in vacuum to prove that the vibrations of strings emitted light, the wave theory of Huygens and Lomonosov failed of immediate acceptance, however. For one thing, it was objected that if light were a wave motion one should be able to see around corners, since waves can bend around obstacles in their path.
We know now that the wave lengths of light waves are so short that the bending, while it does actually take place, is so small that it is not ordinarily observed. As the matter of fact, the bending of a light wave around the edges of an object, a phenomenon known as diffraction, was noted as early as 1665, but the significance of these observations was not realized at the time. It was not until 1827 that the experiments on interference and the measurements of the velocity of light in liquids, at a somewhat later date, demonstrated the existence of optical phenomena for whose explanation a corpuscular theory was inadequate. These experiments enabled the physicists to measure the wave length of the waves and it was proved that the rectilinear propagation of light, as well as the diffraction effects, could be accounted for by the behaviour of waves of short wave lengths.
The next great forward step in the theory of light was made by Hertz who succeeded in producing short wave length waves of undoubted electromagnetic origin and showed that they possessed all the properties of light waves. They could be reflected, refracted, focussed by a lens, polarized, and so on, just as could waves of light. The wave theory of light seemed to have defeated the particle theory. However, in 1888 Stoletov, Russian physicist, discovered the phenomenon of photoelectric emission.
Early in the twentieth century it was found that light could cause atoms to emit electrons and that the energy possessed by the electron very greatly exceeded that which the atom could have received according to electromagnetic-wave theory. It was at this point that the wave theory failed to suggest an explanation,
A return, at least to some extent, to the particle theory of light appeared to be necessary. In 1905 Einstein suggested that the energy of a light beam is concentrated in the form of small particles proportional to the frequency of light. These localized concentrations of energy he called "photons" or "light quanta". Thus, on the one hand, all the phenomena of interference, diffraction and polarization are described by the wave theory. On the other hand, there are many phenomena of the interaction of light with matter in the processes of emission and absorption, which are readily described in terms of photons.
According to the present concept light has a dual character such that it may be represented equally well by waves or by particles. The wave and particle properties of light are found by modern scientists to be two different aspects of the same thing. These two aspects are to be regarded as complementary rather than antogonistic, each being correct when dealing with the phenomena in its own domain.
In the passage of a beam of light through a medium, some
of the radiant energy is absorbed and transformed into heat; some of the radiant energy is also scattered in all directions. Light, because of its electromagnetic character, sets the electrons of the medium into vibration, thus giving up some of its energy. These electrons re-emit some of this energy in the form of radiation, either of the same wave length as the incident radiation, or of different wave lengths. Absorption and scattering of light take place even in the most transparent media such as air and glass. The colour of the sky, for example, is due to the small amount of scattering of sunlight by the molecules of the air. These molecules are more effective in scattering the shorter wave lengths such as the violet and blue light. When we look away from the sun, we see this scattered light, and the sky thus appears blue. If we were in the stratosphere, where there are fewer scattering particles, the sky would appear much darker, almost black. Since blue and violet light are scattered from the direct beam, sunlight should appear redder as it goes through thicker layers of air. It is for this reason that the setting sun looks redder than the moonday sun.
When a beam of light strikes the surface separating one medium from another - for example, the surface between air and glass - some of the light is reflected back into the first medium at the surface of separation and the remainder enters the second medium. The light which passes from one medium into another is said to be refracted. If the surface of separation between the two media is smooth and polished, the light which is thrown back into the first medium is said to be regularly reflected; if the surface is rough, the light is diffusely reflected. Unless otherwise stated, we shall assume that the surface between two media is smooth and polished. In general, smooth, polished metal surfaces will reflect about 90 per cent of the incident light, while smooth polished glass surfaces will reflect from 4 to I0% for angles of incidence from 0° to 60°.
When a beam of light strikes a polished surface separating two media, such as air and glass, part is reflected and part is refracted. The angles of incidence, reflection and refraction are all measured from a normal to the surface; a normal to sur-
face at a given point is a line drawn perpendicular to the surface at that point. The two laws of reflection are as follows: 1)The incident ray, the normal, and the reflected ray all
lie in one plane. 2) The angles of incidence and reflection are equal.
These two laws can easily be verified experimentally. These laws enable us to construct the reflected rays when the incident rays and the position and shape of the reflection surface are given. Rotating mirrors provide an interesting application of the laws of reflection. Such mirrors are used for measuring the deflection of a galvanometer coil and other rotating devices.
When the mirror turns through a small angle, say 1°, the angle
of incidence is increased by 10 and so is the angle of reflection. If the incident beam comes from a stationary source, the reflected beam will travel through 2°. In general, the angle through which the reflected beam rotates is twice the angle through which the mirror rotates.
On looking into a plane reflecting surface one sees, apparently behind the surface, images of any objects that are in front of the surface. Some of the light from each object point is reflected at the surface, and enters the eye as though it were coming from points behind the surface. Apparent images of this kind from which the light is diverging are termed virtual images. When, by reflection or refraction at a curved surface, the light from object point is made to converge again through points, an image is formed that actually exists at the position to which the light converges. Such images, which can be received on a screen, as with the image formed by the lens of a camera or projection lantern, are termed real images. The position of the image of any object before a plane reflecting surface can be determined from the law of reflection, or it may be found by using Huygen's construction to determine the form of the reflected wave front. The image formed by a plane mirror is situated as far behind the mirror as the object is in front, and the line joining the object and image is perpendicular to the plane of the mirror surface. It is a simple matter, therefore, to find graphically the position of the image formed by a plane mirror, and as the reflected light is travelling apparently
from the image, the path of the light by which an eye in a given position sees the image can readily be found. It follows that in shape and size, the image will be an exact reproduction of the object. The point of the object nearest the mirror is represented by the point of the image nearest the mirror, and the top and bottom of the image correspond with the top and bottom of the object, that is the image is erect. The question of whether the image is or is not reversed left for right, or perverted, as it is sometimes called, depends on the position of the observer in viewing both object and image. As may be readily seen by experiment, the extent of image in a plane mirror that can be seen by an eye in any position will depend on the size of the mirror and the position of the eye. This extent of image seen is termed the field of view of the mirror.
When a ray incident in a plane containing the normals of the two mirrors is reflected from two mirrors in succession, it undergoes a total deviation that depends only on the angle between the mirrors. The total deviation produced is, therefore, independent of the angle of incidence at the first mirror and a rotation of the mirrors about an axis perpendicular to the plane containing their normals, and keeping the angle between them constant, produces no change in the final direction of the reflected light. Successive reflection from two mirrors is used in all possible cases where considerable accuracy is required
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