The geometrical construction of images formed by concave mirrors is practically the determination of the foci of points. To make this construction but two rays are necessary for each point, since the intersection of any two reflected rays determines the image of a point. By using the ray passing through C, which is a secondary axis, and the ray parallel to the principal axis, only three A lines need be drawn.

1. When the Object is at an Infinite Distance. - Since the focus of rays parallel to the principal axis is at the principal focus, the image will be at F, and will be a point.
2.  When the Object is at a Finite Distance beyond the Center of Curvature. - Following the rule for construction given above, the points A' and B' (Fig. 1) are found. These being the extreme points of the image, all other points lie between them. The image is real, inverted, smaller than the object, and be- A tween the center of curvature and the focus. ween the center of curvature and the focus.

1.  When the Object is at the Center of Curvature. - In this case, Fig. 2 shows that the image will be real, inverted, of the same size as the object, and at the center of curvature.

1. When the Object is between the Center of Curvature and the Principal Focus. - This is the reverse of case (b), and Fig. 3 shows that the image is real, inverted, larger than the object, and beyond the center of curvature.

1. When the Object is at the Principal Focus. - This is the reverse of case (a). The rays are sent off in parallel lines; hence the image is at an infinite distance away; that is, there is none.
2. When the Object is between the Principal Focus and the Mirror. - In this case, as shown in Fig. 4, the reflected rays are divergent, and there can be no real image. By prolonging these rays behind the mirror, however, the image is seen to be virtual, upright, and larger than the object.

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مواضيع ذات صلة

The Convex Mirror

Umbra and Penumbra

Incandescent Lighting

The Luminous Arc

The Terrestrial Telescope

The Polariscope

The Simple Microscope

The Compound Microscope

The Camera

The Astronomical Telescope

Galileos Telescope the Opera Glass

The Prism Binocular