774 



A MANUAL OF PHYSIOLOGY 



ray which falls upon the reflecting surface (incident ray), and the 

 normal to the surface, are in one plane. The second law is that the 

 reflected ray makes ; with the perpendicular (normal} to the reflecting 

 surface, the same angle as the incident ray. A corollary to this is that 

 a ray perpendicular to the surface is reflected along its own path. 



Reflection from a Plane Mirror. Let a ray of light coming fron? 

 the point P meet the surface DE at B, making an angle PBA with 



the normal to the surface. The re- 

 flected ray BC will make an equal 

 angle ABC with the no-rmal ; and the 

 eye at C will see the image of P as if 

 it were placed at P', the point where 

 the prolongation of BC cuts the 

 straight line drawn from P perpendicu- 

 lar to DE. This is the position of an 

 ordinary looking-glass image. 



Reflection from a Concave Spherical 

 Mirror. A spherical surface may be 

 supposed to be made up of an infinite 

 number of infinitely small plane surfaces. 

 The normal to each of these plane 

 surfaces is the radius of the sphere, and the reflected ray makes with 

 the radius at the point of incidence the same angle as the incident 

 ray. Let D (Fig. 275) be the middle point of the mirror, and C its 

 centre of curvature, i.e., the centre of the sphere of which it is a 



FIG. 274. REFLECTION FROM 

 A PLANE MIRROR. 



FIG. 275. REFLECTION FROM A CONCAVE 

 SPHERICAL MIRROR. 



FIG. 276. FORMATION OF REAL IN- 

 VERTED IMAGE BY A CONCAVE 

 SPHERICAL MIRROR. 



segment. Then CD is the principal axis, and any other line through 

 C which cuts the mirror is a secondary axis. When the mirror is a 

 small portion of a sphere, rays parallel to the principal axis are 

 focussed at the principal focus F midway between C and D ; rays 

 parallel to any secondary axis are focussed in a point lying on that 

 axis ; and rays diverging from a point on any axis are focussed in a 

 point on the same axis. 



These facts afford a simple construction for finding the position of 

 the image of an object formed by a concave mirror. Let AB be the 



