A concave mirror is seen in the accompanying diagram, and may be called the segment of a hollow sphere—V W. The point C is the geometrical centre, and O C the radius; F is the focus; the line passing through it is the optical axis; O being the optical centre. All perpendicular rays pass through C. All rays falling in a direction parallel with the optical axis are reflected and collected at F. Magnified images will be produced, and if the object be placed between the mirror and the focus, the image will appear at the back; while if the object be placed between the geometrical centre and the focus, the image will appear to be in front of the mirror.
We can understand these phenomena by the accompanying diagrams. Suppose a ray A n passes from one object, A B, at right angles, it will be reflected as n A C, the ray A C being reflected to F. These cannot meet in front of the mirror, but they will if produced meet at a, and the point A will be reflected there; similarly B will be reflected at b, and thus a magnified image will appear behind or at the back of the mirror’s surface. In the next diagram the second supposed case will produce the image in the air at a b, and if a sheet of paper be held so that the rays are intercepted, the image will be visible on the sheet. In this case the perpendicular ray, A n, is reflected in the same direction, and the ray, a c, parallel with the axis is reflected to the focus. These rays meet at a and corresponding rays at b, when the image will be reproduced; viz., in front of the mirror.
Fig. 87.—Reflection of mirrors (I).
Fig. 88.—Reflection of mirrors (II).
The concave mirror is used in the manufacture of telescopes, which, with other optical instruments, will be described in their proper places. We will now look at the Refraction of light.
Bodies which permit rays of light to pass through them are termed transparent. Some possess this property more than others, and so long as the light passes through the same medium the direction will remain the same. But if a ray fall upon a body of a different degree of density it cannot proceed in the same direction, and it will be broken or refracted, the angle it makes being termed the angle of refraction.
