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. [Illustration: Fig. 87.—Reflection of mirrors (I).] [Illustration: 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. For instance, a straight stick when plunged into water appears to be broken at the point of immersion. This appearance is caused by the rays of light taking a different direction to our eyes. If in the diagram (fig. 89) our eye were at _o_, and the vessel were empty, we should not see _m_; but when water is poured into the vessel the object will appear higher up at _n_, and all objects under water appear higher than they really are. [Illustration: Fig. 89.—Refraction in water.] [Illustration: Fig. 90.—A water-bottle employed as a convergent lens.]