The analysis of white light into the spectrum colours, and the reformation of the original light by transmitting the spectrum through a reversed prism, proved, to the satisfaction of Newton and subsequent physicists until late in the 19th century, that the various coloured rays were present in white light, and that the action of the prism was merely to sort out the rays. This view, which suffices for the explanation of most phenomena, has now been given up, and the modern view is that the prism or grating really does manufacture the colours, as was held previously to Newton. It appears that white light is a sequence of irregular wave trains which are analysed into series of more regular trains by the prism or grating in a manner comparable with the analytical resolution presented by Fourier’s theorem. The modern view points to the mathematical existence of waves of all wave-lengths in white light, the Newtonian view to the physical existence. Strictly, the term “monochromatic” light is only applicable to light of a single wave-length (which can have no actual existence), but it is commonly used to denote light which cannot be analysed by the instruments at our disposal; for example, with low-power instruments the light emitted by sodium vapour would be regarded as homogeneous or monochromatic, but higher power instruments resolve this light into two components of different wave-lengths, each of which is of a higher degree of homogeneity, and it is not impossible that these rays may be capable of further analysis.
§ 3. Divisions of the Subject.—In the early history of the science of light or optics a twofold division was adopted: Catoptrics (from Gr. κάτοπτρον, a mirror), embracing the phenomena of reflection, i.e. the formation of images by mirrors; and Dioptrics (Gr. διά, through), embracing the phenomena of refraction, i.e. the bending of a ray of light when passing obliquely through the surface dividing two media.[2] A third element, Chromatics (Gr. χρῶμα, colour), was subsequently introduced to include phenomena involving colour transformations, such as the iridescence of mother-of-pearl, feathers, soap-bubbles, oil floating on water, &c. This classification has been discarded (although the terms, particularly “dioptric” and “chromatic,” have survived as adjectives) in favour of a twofold division: geometrical optics and physical optics. Geometrical optics is a mathematical development (mainly effected by geometrical methods) of three laws assumed to be rigorously true: (1) the law of rectilinear propagation, viz. that light travels in straight lines or rays in any homogeneous medium; (2) the law of reflection, viz. that the incident and reflected rays at any point of a surface are equally inclined to, and coplanar with, the normal to the surface at the point of incidence; and (3) the law of refraction, viz. that the incident and refracted rays at a surface dividing two media make angles with the normal to the surface at the point of incidence whose sines are in a ratio (termed the “refractive index”) which is constant for every particular pair of media, and that the incident and refracted rays are coplanar with the normal. 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.
Of the subordinate phases of the science, “physiological optics” is concerned with the phenomena of vision, with the eye as an optical instrument, with colour-perception, and with such allied subjects as the appearance of the eyes of a cat and the luminosity of the glow-worm and firefly; “meteorological optics” includes phenomena occasioned by the atmosphere, such as the rainbow, halo, corona, mirage, twinkling of stars and colour of the sky, and also the effects of atmospheric dust in promoting such brilliant sunsets as were seen after the eruption of Krakatoa; “magneto-optics” investigates the effects of electricity and magnetism on optical properties; “photo-chemistry,” with its more practical development photography, is concerned with the influence of light in effecting chemical action; and the term “applied optics” may be used to denote, on the one hand, the experimental investigation of material for forming optical systems, e.g. the study of glasses with a view to the formation of a glass of specified optical properties (with which may be included such matters as the transparency of rock-salt for the infra-red and of quartz for the ultra-violet rays), and, on the other hand, the application of geometrical and physical investigations to the construction of optical instruments.
§ 4. Arrangement of the Subject.—The following three divisions of this article deal with: (I.) the history of the science of light; (II.) the nature of light; (III.) the velocity of light; but a summary (which does not aim at scientific precision) may here be given to indicate to the reader the inter-relation of the various optical phenomena, those phenomena which are treated in separate articles being shown in larger type.
The simplest subjective phenomena of light are [Colour] and intensity, the measurement of the latter being named [Photometry]. When light falls on a medium, it may be returned by [Reflection] or it may suffer [Absorption]; or it may be transmitted and undergo [Refraction], and, if the light be composite, [Dispersion]; or, as in the case of oil films on water, brilliant colours are seen, an effect which is due to [Interference]. Again, if the rays be transmitted in two directions, as with certain crystals, “double refraction” (see [Refraction, Double]) takes place, and the emergent rays have undergone [Polarization]. A [Shadow] is cast by light falling on an opaque object, the complete theory of which involves the phenomenon of [Diffraction]. 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; instruments serviceable for simultaneous vision with both eyes are termed [Binocular Instruments]; the [Stereoscope] may be placed in this category; the optical action of the Zoétrope, with its modern development the [Cinematograph], depends upon the physiological persistence of [Vision]. Meteorological optical phenomena comprise the [Corona], [Halo], [Mirage], [Rainbow], colour of [Sky] and [Twilight], and also astronomical refraction (see [Refraction, Astronomical]); the complete theory of the corona involves [Diffraction], and atmospheric [Dust] also plays a part in this group of phenomena.
I. History
§ 1. There is reason to believe that the ancients were more familiar with optics than with any other branch of physics; and this may be due to the fact that for a knowledge of external things man is indebted to the sense of vision in a far greater degree than to other senses. That light travels in straight lines—or, in other words, that an object is seen in the direction in which it really lies—must have been realized in very remote times. The antiquity of mirrors points to some acquaintance with the phenomena of reflection, and Layard’s discovery of a convex lens of rock-crystal among the ruins of the palace of Nimrud implies a knowledge of the burning and magnifying powers of this instrument. The Greeks were acquainted with the fundamental law of reflection, viz. the equality of the angles of incidence and reflection; and it was Hero of Alexandria who proved that the path of the ray is the least possible. The lens, as an instrument for magnifying objects or for concentrating rays to effect combustion, was also known. Aristophanes, in the Clouds (c. 424 B.C.), mentions the use of the burning-glass to destroy the writing on a waxed tablet; much later, Pliny describes such glasses as solid balls of rock-crystal or glass, or hollow glass balls filled with water, and Seneca mentions their use by engravers. A treatise on optics (Κατοπτρικά), assigned to Euclid by Proclus and Marinus, shows that the Greeks were acquainted with the production of images by plane, cylindrical and concave and convex spherical mirrors, but it is doubtful whether Euclid was the author, since neither this work nor the Ὀπτικά, a work treating of vision and also assigned to him by Proclus and Marinus, is mentioned by Pappus, and more particularly since the demonstrations do not exhibit the precision of his other writings.
Reflection, or catoptrics, was the key-note of their explanations of optical phenomena; it is to the reflection of solar rays by the air that Aristotle ascribed twilight, and from his observation of the colours formed by light falling on spray, he attributes the rainbow to reflection from drops of rain. Although certain elementary phenomena of refraction had also been noted—such as the apparent bending of an oar at the point where it met the water, and the apparent elevation of a coin in a basin by filling the basin with water—the quantitative law of refraction was unknown; in fact, it was not formulated until the beginning of the 17th century. The analysis of white light into the continuous spectrum of rainbow colours by transmission through a prism was observed by Seneca, who regarded the colours as fictitious, placing them in the same category as the iridescent appearance of the feathers on a pigeon’s neck.
§ 2. The aversion of the Greek thinkers to detailed experimental inquiry stultified the progress of the science; instead of acquiring facts necessary for formulating scientific laws and correcting hypotheses, the Greeks devoted their intellectual energies to philosophizing on the nature of light itself. In their search for a theory the Greeks were mainly concerned with vision—in other words, they sought to determine how an object was seen, and to what its colour was due. Emission theories, involving the conception that light was a stream of concrete particles, were formulated. The Pythagoreans assumed that vision and colour were caused by the bombardment of the eye by minute particles projected from the surface of the object seen. The Platonists subsequently introduced three elements—a stream of particles emitted by the eye (their “divine fire”), which united with the solar rays, and, after the combination had met a stream from the object, returned to the eye and excited vision.
In some form or other the emission theory—that light was a longitudinal propulsion of material particles—dominated optical thought until the beginning of the 19th century. The authority of the Platonists was strong enough to overcome Aristotle’s theory that light was an activity (ἐνέργεια) of a medium which he termed the pellucid (διαφανές); about two thousand years later Newton’s exposition of his corpuscular theory overcame the undulatory hypotheses of Descartes and Huygens; and it was only after the acquisition of new experimental facts that the labours of Thomas Young and Augustin Fresnel indubitably established the wave-theory.