If the outside of a sphere is made the reflecting surface, the mirror is convex. The focus and image are determined as in the case of the concave mirror, except that the reflected rays must be prolonged behind the mirror before they meet, and hence the image is a virtual one. It is also upright and smaller than the object, as a construction like Fig. 1 will show.
Demonstrations. - Using two mirrors, one concave and the other convex…Observe that the real image can be caught on a screen, while the virtual cannot, though both alike can be seen with the mirror. A piece of ground glass forms an excellent screen.
Hang a concave mirror on the wall so that its center is at the height of the eye. Let two students of the same height stand in front of the mirror 10 or 15 feet away. While one watches the image of the second, let the second walk slowly toward the mirror. By careful observation, student No. 1 can see the inverted image of the face of student No. 2 advance from the mirror and finally rest upon the shoulder of No. 2 when he is at the center of curvature.
If the concave mirror used in Fig. 2 is turned over, its opposite side can be used for a convex mirror. Figure 3 shows how a group of parallel rays are reflected as a diverging pencil. The focus of these rays can be found by projecting the outer rays behind the mirror. From this position the focal length can be found; it will be practically half the radius of curvature.
