Geometrical optics / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1913
Catalogue details

Licence: In copyright

Credit: Geometrical optics / by Archibald Stanley Percival. 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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    reflected at K in the direction KE, so that the angle of inci- dence <j> = —<f)r. Produce EK to Q, cutting the axis in Q. Then PKQ is a triangle, of which the exterior angle PKE is bisected by the line CK, that meets the base produced in C. PK PC _ PA + AC _ PA - CA __p - r QK ~ CQ (Euc. vi. A) CA + AQ CA - QA r-q When, therefore, a thin centric pencil is under considera- R Fig. 10. tion, and PK and QK may be regarded as equal to PA and QA, or p and q, we have, as before— P _ V — 9 w]aich reduces to - -f - = - q r — q p q r When the incident rays are parallel, i.e. when p — oc , 1 12 1 - = 0, consequently - = - = -, so they are reflected as if they came from a point F behind the mirror, such that EA = JCA; in other words, the principal focus is virtual and / 112 1 is negative. We see, then, that the old formula —\-- = - = - & P q r f still holds good if we assign the proper negative value to r and /.
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