444 



CONCAVE MIRRORS. 



of the candle, bat of the aperture, and will project the image close to 

 the aperture itself. Try first, by holding a piece of paper at the spot 

 where the mirror is to be placed, whether a bright light is received 

 through the aperture, and afterwards place the mirror so that it 

 may be just in the middle of the bright triangular beam of light seen 

 on the paper. 



Rays which are parallel to the axis of a concave 

 mirror, and are reflected by it, meet after reflection in 

 a point which, as the point / in fig. 250, is exactly as 



FIG. 250. 



far from the middle of the surface of the mirror as from 

 its centre of curvature. This point is called the 

 principal focus of the mirror, and the distance of the 

 focus from the mirror, which is half of the whole length 

 of the radius of curvature, is called the focal length of 

 the mirror. 



Since rays parallel to the axis are reflected to the 

 focus, it is obvious that rays which proceed from the 

 focus are reflected in directions parallel to the axis. 



Rays which proceed from a point near the axis either 

 meet after reflection in a point near the axis, and 

 produce at that point a real image of the luminous 

 point, as in fig. 249, or the rays diverge after reflection, 



