Elements of natural philosophy (Volume 2-3) . being substituted in Equation (37),we obtain o j i 2 (m - 1) 1 H— (42) 50. If in Equation (20), we make r infinite, we get Deviation at aplane surface byrefraction; or, / mf *»/ = /, which answers to the case of a small pencil deviatedat a plane surface separating two media of differentdensities, as air and water. On the supposition that the tensM medium- radiant is in the denser medium, m becomes —, and m this in the preceding Equation gives /= ™>f; that is, to an eye situated without this medium, the dis-tance of the radiant Fi^ 82 from the de
Elements of natural philosophy (Volume 2-3) . being substituted in Equation (37),we obtain o j i 2 (m - 1) 1 H— (42) 50. If in Equation (20), we make r infinite, we get Deviation at aplane surface byrefraction; or, / mf *»/ = /, which answers to the case of a small pencil deviatedat a plane surface separating two media of differentdensities, as air and water. On the supposition that the tensM medium- radiant is in the denser medium, m becomes —, and m this in the preceding Equation gives /= ™>f; that is, to an eye situated without this medium, the dis-tance of the radiant Fi^ 82 from the deviating sur-face will appear dimin-ished in the ratio ofunity to the relative in-dex of refraction of theray in passing from the •denser to the rarer me-dium. This accountsfor the apparent eleva-tion above their true positions of all bodies beneath thesurface of fluids, as the bottom of a vessel partly filledwTith water, and the apparent bending of a straight stickat the surface when partly immersed in the same fluid. Illustration;. Appearancesaccounted for. ELEMENTS OF OPTICS. 211 APPLICATION TO THE DEVIATION OF LIGHTBY SPHERICAL REFLECTORS. § 51. In reflexion, we have only to consider one <le- E(i»ation , t^ • tit • applicable to a viatmg surface. Equation (20) applies here by making spherical concave m = - 1, Which reduces it tO, reflector; 1_ f 2^r 1 7 (43) But two cases can arise, and these are distinguished bythe sign of the radius. The reflector may be concavetowards incident light, in which case r will be positive,or it may be convex towards the same direction, whenr will be negative. Equation (-43) relates to the firstcase, which will now be discussed. If the incident rays be parallel, —- = 0, and / Incident raysparallel; / or. / = 2 r r. = f, 2 Ffe 33.
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