The proportion between the lines D E and G F, when the media concerned are air and water, is approximately as 4 to 3. These proportions, indicated as fractions, are the refractive indices, i.e., A, or 1•33. If, however, the media are considered in an inverse order, that is to say. if the light passes from water to air, the index of refraction is consequently reversed, it would then be 3. (See also Refraction.) Lenses are transparent media, which, from the curvature of their surfaces, have the pro perty of causing the luminous rays which traverse them to either converge or diverge. Accord ing to their curvature they are either spherical, cylindrical, elliptical, or parabolic. Those used in optics are exclusively spherical. They are usually made of crown glass and of flint glass. These two glasses possess different refractive indices. Combination lenses of the two glasses are made. (For detailed information regarding lenses, see under Lens.) Dispersion and avoid confusion we have, on explaining the phenomenon of refraction, avoided mention of the phenomenon known as dispersion. When white light passes from one medium to another it is decomposed into several kinds of light. An experiment proving this can easily be made by boring a small hole in the shutter of a darkened room, to allow of a pencil of the sun's rays to pass through. This pencil will give you a round and colorless image of the sun. If, now, a flint glass prism be interposed in its path, the beam on emerging from it is refracted and produces on a screen a vertical band, rounded at the ends, colored in all the tints of the rainbow. This is termed the solar spectrum, and contains an infinity of different tints all imperceptably merging into each other. For the sake of simplicity, however, it is customary to dis tinguish seven principal colors, these are, violet, indigo, blue, green, yellow, orange. and red. This is the order in which they are arranged in the spectrum, the violet being the most refrangible, and the red the least so. See also Spectrum.
This separation of white light into its component colors sometimes occurs in lenses, and is termed chromatic aberration.
See under Aberration.
Optical the term optical instrument is meant any combination of lenses or mirrors. They are divided by Ganot into three classes, according to the ends they are intended to answer, viz. :—(1) Microscopes which are designed to obtain a magnified image of any object whose real dimensions are too small to admit of its being seen distinctly by the naked eye. (2) Tele scopes by which very distant objects, whether terrestial or celestial, may be observed. (3) Instru ments to project on a screen a magnified or diminished image of any object, which can thereby be either depicted or rendered visible to a crowd of spectators, such as the camera lucida, the camera obscura, photographic apparatus, etc. The two former classes yield virtual images ; the last, with the exception of the camera lucida, yield real images.
The Eye as an Optical structure of the eye may be considered as a camera obscura, of which the pupil is the aperture, the crystalline the condensing lens, and the retina the screen upon which the image is formed. See Vision.
Phosphorescence and Fluorescence.—See under these headings.
Double Refraction, Interference and Polarisation. large number of crystalline bodies possess the property of double refraction, in virtue of which a single incident ray, in passing through any one of them, is divided into two, or undergoes what is termed bifurcation. An object, therefore, viewed through one of these crystals appears double. Iceland spar, crystallized carbonate of calcium, possesses the property in a most remarkable degree. The term interference is given to the reciprocal action which two rays of light exert upon each other when emitted from two neighboring sources, and caused to meet each other under a very different angle. If two similar waves start from the same place, at the same time, they increase each others intensity, and the result is a wave of double light ; but if one wave is half an undulation in advance of the other, the crest of one occupies the position of the hollow of the other, and the result is a dead level. If the intervals of starting are less than half a wave length, the result is a series of smaller waves, the magnitude of which depends upon the distance which one wave has in advance of the other. The interference of the waves of light may be produced in many ways by diffraction, or by reflection, from thin films, such as soap bubbles, oz from minute particles.* Polarization Of light is the state into which the ethereal undulations which cause the sensation of light are brought under certain conditions. These undulations are perpendicular to the line of transmission of the wave, as in a stretched cord, but in a ray of common light appear to take place successively in all directions, in the manner shown in diagram A, Fig. 317 (but with the transitions far more gradual), the vibrations successively passing through rectilinear, elliptical and circular phases with inconceivable rapidity. If now the vibrations become or are rendered stable in any form of orbit, the light is in the condition known as polarized, and the state is one of plane, elliptical or circular polarization, according as the orbit resembles B, C, or D. The most familiar and simple form is that of plane polarization. This may be produced in various ways, the apparatus producing such modifications being termed a polarizer. When produced, however, the effects can only be perceived by exam ining them through another piece of apparatus called the analyser. The two combined with the necessary adjustments, form a polariscope, of which there are many forms.