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In like manner, if the rays pass from the rarer to the denser medium, in the direction Y p, the value of F will be positive, because in this case n will be greater than 1, and consequently the focus will lie to the right of the vertex B, as at F, the rays diverging from it being those which, by refraction, pass into the medium to the left of the surface ABC. The focus F, therefore, in this case, is also imaginary. The same fig 82, therefore, will represent the circumstances attending the case in which the denser medium is at the convex side of the surface, the only difference being that in this latter case F is the focus of the rays Y p, and F the focus of the rays Y p The distances of F and F from B and o respectively will be the same as in the former case. 136 To illustrate the application of the preceding formula, let us suppose, for example, that the denser medium is glass, and the rarer air, and that consequently the value of n, for rays passing from the rarer to the denser, is , and its value for rays passing from the denser to the rarer is. We have, consequently, in the case represented in fig 82, that is to say, the distance of the principal focus of the parallel rays Y p from B is three times the radius o B, and consequently its distance F o from o is twice its radius. prev next |