Velocity of Winds. The Anemometer. Description. Mode of Operation. The Signal Service. Value of the Telegraph. Signal Service a Part of the Army. CHAPTER VIII. OPTICS. SECTION I. GENERAL PRINCIPLES. 369. Definition of Optics. - OPTICS is that branch of Physics which treats of the phenomena of light. 370. Definition of Light.-Light is that physical agent which, acting upon the eye, produces the sensation of sight. 371. Two Theories of Light. Two theories have been advanced to account for the phenomena of light: the Emission Theory, and the Undulatory or Wave Theory. According to the emission theory, light consists of infinitely small particles of matter, shot forth from luminous bodies with immense velocity, which, falling on the retina of the eye, produce the sensation of sight. According to the undulatory theory, light, like heat, is caused by the vibrations of the molecules of bodies. It is transmitted by a highly elastic medium called luminiferous ether. This medium, which also transmits radiant heat, extends through space, penetrates all bodies, and exists in the intervals between their molecules. The molecular vibrations of a luminous body are imparted to the neighboring ether, and are propagated through it by a succession of spherical waves; these waves, falling on the retina of the eye, excite the sensation of sight. Light and radiant heat are very closely related to each other, being forms of radiant energy; they are generated in the same manner and are propagated through the same medium, but they differ from each other in their wave-length, and, as a consequence, in their mode of action on bodies. Heat is produced by waves of greater length than those which cause light. The vibrations of ether also are more rapid in the case of light. In sound the particles of air vibrate to and fro in the direction of propagation; in light and radiant heat the particles of ether vibrate to and fro in a direction perpendicular to that of propagation. In sound the vibrations are longitudinal, or in the direction of the in light and radiant heat they are transversal, or perpendicular to the rays. rays; The idea of transversal vibrations may be illustrated by a rope made fast at one end and held by the hand at the other. If the free end be moved rapidly to and fro, at right angles to the rope, a succession of waves will run along the rope, while the particles of the rope simply vibrate back and forth in perpendiculars to the rope. If a stone be dropped into a pool of still water, a series of waves will be propagated outward, while the particles of water simply rise and fall, their motion being perpendicular to the direction of propagation. The undulatory theory is now generally accepted by physicists. This kind of wave motion is shown in Fig. 235. The white dots represent molecules of ether, and the light is supposed to pass in the direction A B. The distances b'd and c' d' are called wave-lengths, that is, from the crest of one wave to the crest of the next. The distances b'b', f'f", c' c', and d' d' represent amplitudes of vibration. Through these distances the molecules of either oscillate back and forth. 372. Luminous Bodies. Sources of Light. Bodies that emit light are said to be luminous; those that are seen by light derived from others are said to be illuminated. Luminous bodies generate light; illuminated bodies reflect and diffuse it. The sun is a luminous body; the moon is illuminated by it. The principal sources of light are the sun, the stars, heat, chemical combination, phosphorescence, and electricity. The ultimate cause of the sun's light is unknown. The sun is surrounded by a gaseous envelope, called the photosphere, which appears to be in a state of intense ignition. The molecular vibrations of this envelope are undoubtedly the immediate sources of solar light and solar heat. The stars are similar to the sun, but on account of their enormous distances from us, they send us but a small amount of light and heat. If a body be heated its molecules are thrown into vibration, and when its temperature reaches 900° or 1000° F., it begins to be luminous in the dark. Beyond that its brightness increases as its temperature rises. The light developed by chemical combinations is mostly due to the heat that accompanies them. Combustion is an example; the affinity between the oxygen of the air and the carbon of the fuel causes them to rush together under favorable circumstances, thus generating heat and ultimately light itself. Phosphorescence is the property that some bodies, have of giving out light under certain conditions, without heat; it is often observed in decaying animal and vegetable matter, and in some minerals. The light of the fire-fly is an example of this property. Electricity is the source of a species of light that rivals in intensity that of the sun itself. It will be treated of hereafter. 373. Media. - Opaque and Transparent Bodies.A MEDIUM is anything that transmits light; thus, free space, air, water, and glass are media. A medium is said to be homogeneous when the chemical composition and density of all its parts are the same. A TRANSPARENT BODY is one that permits light to pass through it freely; as glass, diamonds, rock-crystal, and water. When bodies permit light to pass through them, but not in such quantity as to allow objects to be seen through them, they are called translucent. Thus, scraped horn, ground glass, oiled paper, and thin porcelain are translucent. An OPAQUE BODY is one that does not permit light to pass through it. Thus, iron, wood, and granite are opaque bodies. No bodies are perfectly opaque; when cut into sufficiently thin leaves, they are more or less translucent. 374. Absorption of Light. -- No body is perfectly transparent; all intercept or absorb more or less light, but some absorb much more than others. If light be transmitted through great thicknesses of media which in thin layers are transparent, a quantity of light is absorbed, and it often happens that the transmitted light is not of sufficient intensity to produce the sensation of sight. The atmosphere seems perfectly transparent, but it is a known fact that much of the light of the sun is absorbed in reaching the earth, as is shown by the greater brilliancy of the stars in the higher regions, as on mountain-tops. In the high regions of the atmosphere objects are more clearly seen than nearer the earth; indeed, so great is the clearness of vision in these regions, that it becomes exceedingly difficult to judge of distances. Opaque bodies absorb all of the light falling upon them which is not reflected. The physical cause of absorption of light by bodies is some peculiarity of molecular constitution which breaks up and neutralizes the waves of light that enter them. - Pencils. Beams. Propa 375. Rays of Light. gation of Light. A RAY OF LIGHT is a line along which light is propagated. It is perpendicular to the advancing wave-front. When the source is very distant the wave-fronts are sensibly plane and the rays parallel. A PENCIL OF RAYS is a small group of rays meeting in a common point, such as the rays proceeding from a candle or a lamp. When the rays proceed from a common point, they are said to be divergent. When they proceed towards a common point, they are said to be convergent. |