306 PHYSIOLOGICAL PHYSICS. [Chap. xxiv. 



also virtual, whose position varies as in concave 

 mirrors, with that of the source. 



Formation of images in spherical mirrors. 



1. Concave mirrors. Let MN (Fig. 132) be a concave 

 mirror, p its principal focus, and in front of it, beyond 

 its centre of curvature c, let an object (the arrow 

 AB) be placed ; how may the position of the reflected 

 image of the arrow be found ? First draw the principal 

 axis 00, and secondary axis from A and B, namely, 

 AK and BP. From A let fall on the mirror the incident 

 rays AE and AG, and from B incident rays BN and BL. 

 These rays are reflected. The reflected rays of A will 

 meet at a point a on the secondary axis AK, and those 

 of B at a point b on the secondary axis BP. The point 

 a is thus the conjugate focus of A, and an image 

 of the point A is formed there, while b is the conju- 

 gate focus of B, and an image of B is formed there. 

 Similarly, conjugate foci of all points between A and B 

 will l)e formed between a and b, and thus between 

 a and b an image of AB will be formed. The image 

 is in front of the mirror, it is upside down, and is 

 smaller than AB. Thus in concave mirrors, where the 

 object is beyond the centre of curvature, an image will 

 be formed between the centre of curvature and the 

 principal focus, and the image is real, inverted, and 

 smaller than the object. Suppose the object were at 

 an infinite distance, the image would be at the prin- 

 cipal focus. As the object approaches the centre of 

 curvature, the image moves outwards from the prin- 

 cipal focus towards the centre. If the object were at 

 the centre, the image would coincide with it. This 

 will be understood from what has been said about 

 conjugate foci (page 304). For the same reasons it will 

 be understood that should the object be within the 

 centre of curvature, the image will be formed beyond 

 it. Thus, should ab (Fig. 132) be the object, the rays 

 aE are now incident, and GA EA are the reflected 



