REFLECTION OF LIGHT. 



the centre of curvature, to the mirror. This line represents one of 

 the infinite number of rays sent from 8 to the mirror. As this 

 incident ray is perpendicular to the mirror, the reflected ray will 

 coincide with it. (Angles of incidence and of reflection = 0.) The 

 conjugate focus must therefore lie in a line drawn through 8 and C. 

 Draw a line representing some other ray, as Si. From i, the point 

 of incidence, draw the dotted perpendicular iC. Construct the 

 angle Cis equal to the angle CiS. Then will is represent the direc- 

 tion of the reflected ray. The focus must also lie in this line. The 

 intersection of this line with the line drawn through SC marks the 

 position of *, the conjugate focus of 8. 



(2.) If the reflected rays be parallel, of course no focus can be 

 formed. If they be divergent, produce them back of the mirror as 

 dotted lines (Fig. 237) until they intersect. In this case the focus 

 will be virtual, because the rays only seem to meet. In the other 

 cases the focus was real, because the rays actually did meet. 



FIG. 287. 



(8.) With a radius of 4 cm., describe ten arcs of small aperture to 

 represent the sections of spherical concave mirrors. Mark the 

 centres of curvature and principal foci, and draw the principal 

 axes. Find the conjugate foci for points in the principal axis 

 designated as follows : (1.) At a distance of 1 cm. from the mirror. 

 (3) Two cm. from the mirror. (3.) Three cm. from the mirror. 

 (4.) Four cm. from the mirror. (5.) Six cm. from the mirror. 

 Make five similar constructions for points not in the principal axia 

 Notice that each effect is in consequence of the equality between 

 the angle of incidence and the angle of reflection. 



6O4. Formation of Images. Concave mirrors 

 give rise to two kinds of images, real and virtual. After 



