half of the radius.We shall denote the point by F and the focal length by f. A similar proof will apply to the case of a convex mirror,the principal focus will however be, as shewn in fig. 65, behindthe mirror and at a distance from the mirror equal to half theradius. 51. Convention as to signs. If… … Fig. 65. can express this distinction between the two cases if we agreeto distinguish lines drawn from A in opposite directions byopposite signs. The usual convention is that lines drawnto the right from A are called positive, those drawn to theleft negative; if then / and r are the numerical…
Heat · Light
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