30
40. When an object is placed between two plane mirrors, in the first place it is reflected at each of them, which produces two images; these are again reflected at the mirrors to which they are opposite, and thus there is formed an infinite number of images growing more and more distant and more indistinct on account of the light which is lost at each reflexion.
To make this plainer, let O (Fig. 36.) be an object considered as a point placed between the two mirrors XY, ZV. AB a line through O perpendicular to the two mirrors.
| Then there is first a reflexion at A, | which gives an image O′, |
| then a reflexion of this image at B, | which gives another image O″, |
| then a reflextion„ of O″ at A, | which gives another image„ O‴ |
and so on.
| Again, there is a reflexion at B, giving an image O′, | |
| a reflexion of this at A, | giving another image O″, |
| a reflexion„ of O″ at B, | giving another image„ O‴, |
and so on.
An eye placed any where between the mirrors as at E, will see all these images in the directions EO′, EO″, …, Eo′, Eo″, ….
The reader may perhaps find some difficulty in understanding how the image O′, for instance, can be reflected at B, when it is behind the mirror XY, so that no light could come from O′ to B; but he has only to remember, that the light never goes from between the mirrors; O′, O″, &c. are merely imaginary points, where the rays intersect the line AB.[1]
The distances OO′, OO″, &c. are easily calculated. If we put a for OA, b for OB, and c for AB or a+b,
- ↑ If the mirror be inclined to the line of the body, its length may of course be easily calculated by Trigonometry. The data required are the height of the body and of the eye, and those which determine the position of the mirror.
