Geometrical optics / by Val H. Mackinney and Harry L. Taylor.

  • Mackinney, Val H., 1791-1868.
Date:
1908
Catalogue details

Licence: In copyright

Credit: Geometrical optics / by Val H. Mackinney and Harry L. Taylor. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    of the receiving screen from tlie source ; then, from the hiw of inverse squares we have (1) I = wlien the receiving screen faces the source normally. (2) I = cos a where a is the angle of turning of the surface from its ])Osition wlien it faced the source normally. Tlie quantity of liglit Q received by any surface is given by Lambert's equation (3) Q = B ^'jt~ ^^^^ p ^'^^ ^'o, where and A.^ are the areas of the luminous soui-ce and screen respectively, d the distance between the centres of the two surfaces, and a ^ and a.^ the angles made by tlie imaginary linc^ joining tlic two surface centres with their res})ective normals. (Fig. 5.) V-'''«. _ _ _ I !<___- d Fig. 5. All tliese equations (i) (2) and (3) are oidy correct wlieu the dimensions of the luminous source are small as compared witli the distance (/. Tn practice equation (1) is of great use, for by means of it we are able, with a photometer, to compare the intrinsic brilliancy of two light sources, by reducing to equal illumination the light given out by tliem. The essentials of the photometric process are first, the production of two identical surfaces (or one surface), illuminated separately from each source ; and secondly, some means of obtaining equality of dlumination on the said identical surfaces (or one surface). The Aberration of Liglit. Owing to the facts tliat light travels with a finite velocity, and that the eartli has a certain jjcriod of revolution round the sun, light emitted by the heavenly bodies will, more often than not, appear to have proceeded along a different path from what it really has, and we see them in a false position ; hence the term aberration of liglit. The Laws of liefectiou. 1. The incident and reflected paths of the waves of light lie in the same plane witli the normal to the surface at the point of incidence^ and are on opposite sides of the normal.