REFLECTION OF LIGHT. 



former case, the light striking at I or a part of it, would be dispersed in every 

 direction above the surface A B, so as to render the point I visible to an eye 

 placed anywhere in the space above A B. But such is not the case when the 

 surface A B is perfectly smooth and polished. In that case, the light proceed- 

 ing from S and striking on I, will be reflected only in one direction, viz., as if 

 jt came from a point D as far behind A B as S is before it. Thus if we draw 

 S A at right angles to A B, and continue it until A D is equal to A S, then the 

 light will be reflected along I O as if it came from D. 



As a consequence of this, it follows that the incident light S I and the re- 

 flected light I O make equal angles with the reflecting surface A B. 



This is a universal and very important law of optics, and is usually ex- 

 pressed thus : 



When a ray of light falls on a perfectly polished, reflecting surface, it is 

 so reflected that the angle of reflection shall be equal to the angle of incidence. 

 In the diagram, A I S is the angle of incidence, and I B is the angle of re- 

 flection. 



But if a surface such as A B, fig. 2, be exposed to a source of light, it is not one 



Fig. 2. 



point, but every point of it. that will be illuminated. Rays in fact will diverge 

 from S, and will strike upon all points of A B. From what has been already 

 stated, it will be apparent that, after reflection, they will each of them proceed 

 as if they had originally diverged from D. The effect, therefore, ol the re- 

 flecting surface A B will be to convert a pencil of rays, which diverges from ' 



