Ocular refraction and the shadow test . it will bereflected from all points upon the surface of the mirror back to its i8 O C U L A R K K F K A C T I () N. source; the path of each ray will be a radius of the mirror and will beperpendicular to the small plane at the point of incidence; thus, theincident and reflected rays traverse the same path. Any ray passingthrough the centre of curvature of a concave mirror and incident atany point upon the mirror other than the vertex, is reflected to itssource and its path is said to be a secondary axis of the mirror. Figure 14 represents a concavr mirro
Ocular refraction and the shadow test . it will bereflected from all points upon the surface of the mirror back to its i8 O C U L A R K K F K A C T I () N. source; the path of each ray will be a radius of the mirror and will beperpendicular to the small plane at the point of incidence; thus, theincident and reflected rays traverse the same path. Any ray passingthrough the centre of curvature of a concave mirror and incident atany point upon the mirror other than the vertex, is reflected to itssource and its path is said to be a secondary axis of the mirror. Figure 14 represents a concavr mirror A B ; from the points E andF located at equal distances from the vertex upon A B, draw the radiiand locate the centre C and the principle axis D C. Suppose parallelrays strike the mirror, one ray traversing the principle axis, the rayR, incident at E, and S at F, form the angles of incidence R E C andS F C; constructing the angles of reflection CEP and C F P, it isfound that the reflected rays meet at a point P on the principle axis. lei rays by which is called \.h& principle focus of the mirror; it is located half waybetween the centre of curvature and the vertex. By this it is provedthat any ray from an object, parallel to the principle axis of a concavemirror, by reflection passes through the principle focus. The distancebetween the vertex and the principle focus is the measure of ihe focallength of the mirror. A concave mirror renders parallel rays con-vergent, and light situated at the focal point is projected parallel byreflection. The focus then is a point at wliich light rays that divergefrom one point meet again after reflection. Figure 15 represents a concave mirror A B; the light (radiantpoint) is situated at the point G upon the principle axis which is be-yond the centre of curvature C; from the points E and F located uponA 1^, equally distant from the vertex 1), construct the angle of inci- dence G E C and G F C, for the rays G E and G F, and the angles ofreflect
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