Elementary natural philosophy : designed for high schools and academies (1881)
Griffin, La Roy F. (La Roy Freese), 1844-1916 · Philadelphia : Sower, Potts & Co. · leaf 214
irtual* 709. Foci of Concave Lenses. Concave lenses give onlyvirtual foci. Fofcus of Parallel Rays. The focus is found as withconvex lenses, by drawing the perpendiculars and tracing the rays. Thus in Fig. 235, the incident ray-SI is first retracted to G and thencein the direction GH. So the rayS K… … 710. If the rays proceed fromany point, as L, Pig. 236, therefracted rays become more di-vergent, so that the focus is atI. This point is between the lensand the principal focus, and this Fig. 236. focus is also virtual, while the luminous point and thefocus are much nearer than before.
Physics
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