XI 



LIGHT 



167 



<-n,-rature. The distance from this point to the reflecting sur- 

 face is the radius of curvature. Thus in Fig. 77 c is the centre 

 of curvature and cM, cd, cM', are all radii of curvature. 

 MM' is called the diameter or aperture of the mirror and d 

 is called by many names, perhaps pole of the mirror is the 

 best. A line going through the pole and centre of curvature 

 is the principal axis of the mirror, any other radius produced 

 being a secondary axis. We know from geometry that every 

 radius is at right angles to the tangent at the point where it 

 cuts the circle, and since we can consider the tangent and 

 circle as coincident at this point ; from what has been already said 

 it will be clear that the radii are normals to the mirror. 

 Evidently, then, if we place a luminous object at the centre of 

 curvature we shall have all the rays of light from it reflected 



Fio. 77. Reflection from a Spherical Mirror. 



back along the lines of incidence, or the image will be formed 

 at the same place as the object. 



EXPT. 164. Procure a concave mirror and cover it with black 

 paper, except a small part at the centre or round the pole. 

 That is, let the aperture of the mirror be small. Allow rays of 

 sunlight to fall upon it (these come from so great a distance 

 that they can be considered parallel}. Move a very small 

 paper screen up and down in front of the reflecting surface 

 so as not to cut oft' the incident rays. Notice that at a certain 

 point a clear image of the sun is formed, and probably the 

 screen will be burnt. 



The point so obtained is called the principal focus of the 

 mirror. In Fig. 78, F represents this point and C the centre of 

 curvature. The parallel lines show the direction of the sun's 



