ELEMENTARY GENERAL SCIENCE 



CHAP. 



2. The angle of incidence is equal to the angle of reflec- 

 tion. 



The student must bear in mind that these laws apply equally 

 to all forms of radiation, whether in the ether or in air ; i.e., 

 to all ether waves and to all sound waves. 



It was also learnt from Expt. 162 that when a wave strikes a 

 reflecting surface normally, i.e., having travelled along the 

 normal, it is reflected back along the same line. 



Formation of an Image by a Plane Mirror. These two rules 

 enable the formation of an image by a plane mirror to be easily 

 understood. 



Let MM be the plane mirror, and A a bright spot of light like 

 the head of the pin in Expt. 162. First see what happens to the 



light ray which leaves A and 



C strikes the mirror normally. 



It is reflected back along the 

 same line and the reflected 

 ray appears to come from a 

 point A' along A' A. In 

 the case of any other rays, 

 such as AB, the light ray 

 is reflected in such a way 

 that the angle of reflection 

 CBD is equal to the angle 

 of incidence ABC and ap- 

 pears to an eye placed as 

 in Fig. 76 to come along 

 BD from a point A', where 

 BD produced meets AA'. 

 If we make the same con- 

 struction for any other ray AB' it will be reflected and appear 

 to be coming along B'D', which produced backwards will pass 

 through the same point A'. A' is therefore the image of A, 

 and it can be easily proved by geometry that A' is as far behind 

 the mirror as A is in front of it. 



Reflection from Spherical Mirrors. A spherical mirror is 

 a part of a spherical surface which has the power of reflecting. 

 It may be either concave or convex, the former if the reflection 

 takes place from the hollow side, the latter if from the bulging 

 side. The centre of the sphere of which it forms part will 

 evidently be the centre from which the part of it constituting 

 the mirror was struck, and this point is called the centre of 



Fia. 76. The Formation of an Image by a 

 Plane Mirror. 



