a pencil ofparallel rays. A pencil of rays falling on a mirror parallelto its axis is brought to a focus at the principal focus, henceconversely, if a luminous point be placed at the principalfocus the reflected rays will all be parallel to the axis. Itshould be noticed however that to secure this… … Fig. 78. but rays diverging from F will not, after reflexion, be parallelto OF but to OF. For join OF meeting the mirror in A, aray from F in the direction FA is reflected back along itself,moreover F is very approximately half way between 0 andA, it is practically the principal focus of a mirror…
Heat · Light
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