Light (optics)

“ They are: 'optical refraction', 'light-ray', and 'light-velocity' - the latter two serving to explain the first. In a science of optics which seeks its foundation in the intercourse between man's own visual activity and the doings and sufferings of light, these three concepts must needs undergo a decisive change, both in their meaning and in their value for the description of the relevant optical phenomena. For they are all purely kinematic concepts typical of the onlooker-way of conceiving things - concepts, that is, to which nothing corresponds in the realm of the actual phenomena. ”

“ We have also said, that the reflection of the heating rays was governed by similar laws to those influencing the rays of light; consequently, by directing a pencil of sunlight upon the surface of a concave mirror, we obtain the maximum of light and heat at the focal point.
Many modern writers give the ancients too little credit for their knowledge of optical principles, and late investigations seem to prove that the old school of philosophers were much more learned in these matters than has been generally supposed. ”

“ Light is an electromagnetic disturbance of the ether. Optics is a branch of electricity. Outstanding problems in optics are being rapidly solved, now that we have the means of definitely exciting light with a full perception of what we are doing, and of the precise mode of its vibration. ”

“ The research-field of optics includes the investigation of the rays which we have just enumerated. A delimitation may then be made, inasmuch as luminous sources yield no other radiations, and also since the next series of waves, the electromagnetic waves, have a minimum wave-length of 6 mm.
§ 2. The commonest subjective phenomena of light are colour and visibility, i.e. why are some bodies visible and others not, or, in other words, what is the physical significance of the words “transparency,” “colour” and “visibility.” ”

“ In other cases the reflexion only changes the foci from which the rays diverge, and it is only by being produced backwards, that the rays can be said to meet in certain points.
38. There are some cases in which an optical image does in fact present the appearance of something real. If a candle be held before a concave mirror in a dark room, there will often be seen an appearance of an inverted flame in the air, owing to the reflected light converging to foci in front of the mirror, and there illuminating strongly the particles of dust floating about in the room. ”

“ You charge a Leyden jar and let it discharge: vibration follows and light is excited.
It is light, just as good as any other light. It travels at the same pace, it is reflected and refracted according to the same laws; every experiment known to optics can be performed with this etherial radiation electrically produced,—and yet you cannot see it. Why not? For no fault of the light, the fault (if there be a fault) is in the eye. The retina is incompetent to respond to these vibrations—they are too slow. ”

“ It should be observed that at the surface of bodies the refractive power is exerted, and that the light proceeds in a straight line until leaving the body. The refraction is more or less, and in all cases in proportion as the rays fall more or less obliquely on the refracting surface. It is this law of optics which has given rise to the lenses in our camera tubes, by which means we are enabled to secure a well-delineated representation of any object we choose to picture. ”

“ The modern theory of optics represents the plane of polarization of light as depending, not on the position in which the axes of the luminiferous particles lie, but on the direction of those transverse vibrations in which light consists. This theory is, as we have stated in the History, recommended by an extraordinary series of successes in accounting for the phenomena. And this hypothesis of transverse vibrations shows us another mechanical mode, (besides the hypothesis of particles with axes,) by which we may represent the polarity of a ray. ”

“ Physical optics, on the other hand, has for its ultimate object the elucidation of the question: what is light? It investigates the nature of the rays themselves, and, in addition to determining the validity of the axioms of geometrical optics, embraces phenomena for the explanation of which an expansion of these assumptions is necessary. ”

“ When merely common lenses are used in this telescope, the eye glass is limited in power by the necessary confusion in the first image from the aberrations, which become very sensible when that image is much magnified, particularly those arising from the unequal refraction of the light. ”

“ It is hardly necessary to occupy time with experiments in showing that a ray of light can be reflected and refracted by mirrors and prisms, and converged or diverged by transparent lenses. These are simple optical facts, and if you are not familiar with them it will be easy for you to make their acquaintance by studying any simple book upon optics; but I should like to draw your attention to the fact that, in addition to rays of light and electric radiation, we are acquainted with another kind of radiation, which is also susceptible of being refracted, and that is commonly called dark heat. ”

“ Upon the whole we may collect that if a small luminous body be placed before a spherical concave mirror, at some distance from it, the distance of the focus will always be something more than half the radius of the surface, which is its accurate value for the light of the Sun, the rays of which are considered as parallel ”

“ When a beam of light is passed through a hole of, let us say, the eighth of an inch in diameter and decomposed by a prism, the spectrum so produced is imperfect, inasmuch as an infinite number of spectra are thus superposed, and for this reason, that the rays of light entering on the right side of the aperture will give a spectrum falling in a different place to that formed by the rays entering on the left. ”

“ This evil may be remedied, in the first place, by supplying the fault of the pupil, that is, by stopping the extreme rays, and admitting the others through a small aperture in a plate of some opaque substance. This accordingly is found to answer pretty well in some cases, but only where there is a good deal of light proceeding from the object.
In order to explain this more completely and correctly, we will suppose PQR, Fig. 170, to be a small object, 1/20 of an inch long, for instance, which is to be examined by a person who cannot see distinctly any thing nearer to his eye than 6 inches. ”

“ Every ray on striking the surface of the lens is refracted inwards, until it meets with its companions at 129the focus F, in accordance with the law of refraction, by which a ray of light passing from one transparent medium, such as air, to another which in this instance is glass, becomes refracted or bent in proportion to the relative density of the two mediæ. ”

“ A lamplight, for example, sets in motion waves of ether, which pass in through the pupil of the eye, to the retina, where the rods and cones transmit the vibration through the optic nerve to the brain, and then the mind perceives the light. (Note, p. 236.)
THE USE OF THE CRYSTALLINE LENS. [Footnote: The uses of the eye and ear are dependent upon the principles of Optics and Acoustics. They are therefore best treated in Physics.] —A convex lens, as a common burning glass, bends the rays of light which pass through it, so that they meet at a point called the focus. ”

“ It is somewhat singular that no substance has yet been found that will increase the refrangibility of the dark rays beyond the red end of the spectrum. There are many artificial flames which produce this dark light (if we may use such a paradoxical expression) in greater quantity than the sun, whose light is no doubt greatly deteriorated in this respect during its passage through the atmosphere. The substance of which the prism is made also greatly influences the length of the invisible portion of the spectrum. ”

“ Another interesting example of optical illusion is the luminous sensation produced internally when the eye, or the neighbouring parts, are struck or stimulated by friction or electricity. These appearances are experienced even by those who have lost their sight. Müller states that a case was submitted to a legal tribunal to decide whether the luminous sensations which are perceptible when we rub our eyes are really light. ”

“ Physical optics, which was developed during the 19th century, in order to account for interference and diffraction represents light as a wave. (I have no idea if this is correct.) Awadewit (talk) 02:43, 14 April 2009 (UTC) How about, Developed during the nineteenth century, physical optics accounts for diffraction and interference by modeling light as a wave.? User:ScienceApologist Adopted your version. Awadewit (talk) 19:11, 18 April 2009 (UTC) Together, geometrical and physical optics encompass classical optics, a system of models capable of describing and predicting many optical phenomena. ”

“ Lenses—The image cast by a convex lens—Focus—Relative position of object and lens—Correction of lenses for colour—Spherical aberration—Distortion of image—The human eye—The use of spectacles—The blind spot.
Light is a third form of that energy of which we have already treated two manifestations—heat and electricity. The distinguishing characteristic of ether light-waves is their extreme rapidity of vibration, which has been calculated to range from 700 billion movements per second for violet rays to 400 billion for red rays. ”

“ When such a lamp was looked at through a lens, concave or convex, of about six inches focus, from a distance of a few feet, the roughly oval patch of luminosity formed upon the retina, instead of being a mere ill-defined blur, such as would be produced if the transparent media of the eye were composed of homogeneous substances like glass or water, appeared to be made up of a crowd of separate images of the filament, some being brighter than others, as is shown in the diagram Fig. ”

“ In such a telescope of any considerable aperture the image of a bright star at focus is surrounded by a halo of bluish or violet-coloured light,—a defect which is unavoidable in an object-glass composed of a crown and flint lens on account of the irrationality of their spectra (Light, vol. xiv. p. 592) . There seems to be no doubt that different eyes are differently impressed by rays of different wave-length. ”

“ That is, we make eclipses of opaque shades revolving round the usual lighting apparatus, and these we can vary so as to show 5, 10, 15, 20 or 30 seconds of darkness with similar or greater intervals of bright light. We may thus use altogether white or bright lights, which have the greatest power to work in such a manner that one can be easily distinguished from another. ”

“ Some substances have the property of transforming luminous radiations, presenting the phenomena of Calorescence, Fluorescence and Phosphorescence. An optical system is composed of any number of Mirrors or Lenses, or of both. If light falling on a system be not brought to a focus, i.e. if all the emergent rays be not concurrent, we are presented with a Caustic and an Aberration. An optical instrument is simply the setting up of an optical system, the Telescope, Microscope, Objective, optical Lantern, Camera Lucida, Camera Obscura and the Kaleidoscope are examples ”

“ The real cause of the luminosity of the eyes of animals in the dark is now thoroughly understood by physiological opticists and by many practical oculists, and depends upon the well-demonstrated laws of the refraction and reflection of light. For a clear apprehension of the phenomenon, however, it is necessary to understand the properties of the eye as an optical instrument. ”

“ To study the phenomena of diffraction it is necessary that our source of light should be a physical point, or a fine line; for when a luminous surface is employed, the waves issuing from different points of the surface obscure and neutralize each other. A point of light of high intensity is obtained by admitting the parallel rays of the sun through an aperture in a window-shutter, and concentrating the beam by a lens of short focus. The small solar image at the focus constitutes a suitable point of light. ”

“ The outer lens of the objective, designated the ‘front lens,’ is in form a plano-convex lens. The inner lens, called the ‘back lens,’ consists of two separate lenses, one an unequi-convex lens, the other a meniscus-concave lens. This combination gives a beautifully-defined picture on the screen, with flatness of field and abundance of light.
The lamp is entirely distinct from the body, and will burn equally well either outside or inside the body of the lantern, the chimney being attached to the lamp and not to the body. The cistern of the lamp forms its base, and is filled with mineral oil. ”

“ Now those colours have become lately of the greatest value to astronomers, but at first they were merely a nuisance; they interfered with the use of lenses in telescopes. A lens is a piece of glass with curved surfaces; when rays of light strike those surfaces they are bent to a focus (Fig. 27) , as is wanted for making a telescope. But the bending introduces colour which is not wanted; and yet they could not in early days get rid of the colours without also getting rid of the bending to focus. ”

“ At N is a tiny transparent scale of wave lengths illuminated by a faint light reflected from the mirror O, and in the focus of the little lens R, which transfers it by reflection from the front face of the prism to the eye, alongside the edge of the spectrum. ”