Page:A treatise on optics.djvu/25

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CHAP. II. IMAGES FORMED BY MIRRORS. 23

pass in straight lines through the aperture A, and fall upon the paper E F at r. In like manner the green light from G will fall upon the paper at g, and the blue light from B will fall upon the paper at b; thus painting upon the paper an inverted image, rb, of the object, R. B. As every colored point in the object RB has a colored point corresponding to it, and opposite to it on the paper E F, the image br will be an accurate picture of the object R B, provided the aperture A is very small. But if we increase the aperture, the image will become less distinct; and it will be nearly obliterated when the aperture is large. The reason of this is, that, with a large aperture, two adjacent points of the object will throw their light on the same point of the paper, and thus create confusion in the image.

It is obvious from fig. 13., that the size of the image br will increase with the distance of the paper E F behind the hole A. If Ag is equal to A G, the image will be equal to the object; if Ag is less than A G, the image will be less than the object; and if Ag is greater than A G, the image will be greater than the object.

As each point of an object throws out rays in all directions, it is manifest that those only which fall upon the small aperture at A concur in forming the image br; and as the number of these rays is very small, the image br must have very little light, and therefore cannot be used for any optical purposes. This evil is completely remedied in the formation of images by mirrors and lenses.

(24.) Formation of images by concave mirrors. Let A B, fig. 14., be a concave mirror whose centre is C, and let M N be an object placed at some distance before it. Of all the rays emitted in every direction by the point M, the mirror receives only those which lie between M A and M B, or a cone of rays MA B whose base is the spherical mirror, the section of which is A B. If we draw the reflected rays A m, B for all the incident rays M A, M B, by the methods already described, we shall find that they will all meet at the point m,

Fig. 14.

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