EMILY. I have often heard of the surprising effects of burning mirrors, and I am quite delighted to understand their nature. CAROLINE. It cannot be the true focus of the mirror at which the rays of the sun unite; for, as they proceed from a point, they must fall divergent upon the mirror. MRS. B. Strictly speaking, they certainly do. But when rays come from such an immense distance as the sun, their divergence is so trifling as to be imperceptible; and they may be considered as parallel; their point of union is, therefore, the true focus of the mirror, and there the image of the object is represented. Now that I have removed the mirror out of the influence of the sun's rays, if I place a burning taper in the focus, how will its light be reflected? (Fig. 6.) CAROLINE. That, I confess, I cannot say. MRS. B. The ray which falls in the direction of the axis of the mirror is reflected back in the same line; but let us draw two other rays from the focus, fall ing on the mirror at B and F; the dotted lines are perpendicular to those points, and the two rays will therefore be reflected to A and E. CAROLINE. Oh, now I understand it clearly. The rays which proceed from a light placed in the focus of a concave mirror fall divergent upon it, and will be reflected parallel. It is exactly the reverse of the former experiment, in which the sun's rays fell parallel on the mirror, and were reflected to a focus. MRS. B. Yes. When the incident rays are parallel, the reflected rays converge to a focus; when, on the contrary, the incident rays proceed from the focus, they are reflected parallel. This is an important law of optics; and since you are now acquainted with the principles on which it is founded, I hope that you will not forget it. CAROLINE. I am sure that we shall not. But, Mrs. B., you said that the image was formed in the focus of a concave mirror; yet I have frequently seen glass concave mirrors, where the object has been represented within the mirror, in the same manner as in a convex mirror. MRS. B. That is the case only when the object is placed between the mirror and its focus; the image then appears magnified behind, or, as you call it, within the mirror. CAROLINE. I do not understand why the image should be larger than the object. MRS. B. Since rays proceeding from the focus are reflected parallel to each other, rays proceeding from a point still nearer the mirror, and therefore diverging more rapidly before reflection, will continue to diverge after reflection, although less than previously to it. They will therefore proceed after reflection as if they diverged from some point. behind the mirror; and, as in the case of a convex mirror, this point will be in the line joining the centre of the mirror, and the point from which the rays originally proceed; in other words, there will be an image there. The situation of the centre of the mirror being in this case before the mirror, and the image behind it, the image will be situated in this line produced, or prolonged through the mirror. EMILY. Exactly in the same manner as in the case of the convex mirror, the image of the whole object will be made up of the images of the several points which constitute it. MRS. B. Yes. Now, if (fig. 7.) X Y represent a concave mirror; O, the centre of the sphere of which it is part; F, its focus; and A B, an object placed between the focus and the mirror; rays proceeding from A will, after reflection, proceed as if diverging from a point, a, in the line O A produced; that is, there will be an image of A at a. In the same manner there will be an image of B at b; and of the whole body A B between a and b. The lines OA a, O B b, diverge from the point O, and the corresponding points in them are therefore farther from each other in proportion as they are farther from O. The image a b, therefore, being more distant than the object A B from O, is larger than the object. You now, I hope, understand the reflection of light by opaque bodies. At our next meeting, we shall enter upon another property of light, no less interesting, which is called refraction. 391 CONVERSATION XVII. ON REFRACTION AND COLOURS. TRANSMISSION OF LIGHT BY TRANSPARENT BODIES. REFRACTION OF THE ATMO REFRACTION. SPHERE. REFRACTION OF A LENS.-REFRACTION OF THE PRISM.- OF THE COLOURS OF RAYS OF LIGHT. OF THE COLOURS OF BODIES. : MRS. B. THE refraction of light will furnish the subject of to-day's lesson. CAROLINE. That is a property of light of which I have not the faintest idea. MRS. B. It is the effect which transparent mediums produce on light in its passage through them. Opaque bodies, you know, reflect the rays, and transparent |