574 



REFLECTION AND REFRACTION 



[Cn. XIV 



FIG. 311. SEMI-REGULAR REFLECTION. 



Light striking some surfaces is scattered 

 unequally, being reflected to a greater extent 

 in one direction than in others. This repre- 

 sents the kind of reflection produced by me- 

 tallic-faced screens. 



798. Semi-regular 

 reflection. This occurs 

 when a surface is imper- 

 fectly polished, that is, if 

 the surface is an irregular 

 one but not sufficiently 

 irregular to scatter the 

 light equally in all direc- 

 tions. The most famil- 

 iar example is a surface 

 coated with silver or 

 aluminum powder. Here 

 the individual metal par- 

 ticles reflect the light 



regularly, but the different particles lie at different angles and 

 reflect the light in different directions. The result is that 

 the light is scattered in all directions, but the greater part of 

 the light is reflected in the same general direction as it would be if 

 the surface were perfectly polished. This is shown in fig. 247, 311, 

 where the length or number of the rays after reflection indicates 

 the amount of light reflected in that direction. 



799. The use of this semi-regular reflection in projection is 

 in the metallic screens or mirror screens sometimes used in long, 

 narrow auditoriums. Such screens do not appear equally bright 

 when seen from all parts of the room but appear brightest when 

 seen in the direction of the regular reflection from the lantern, that 

 is, when the observer is nearly in line with the lantern, and they 

 appear very dim when seen from the sides of the room, 1 5 or more 

 from the axis of the lantern ( 630). 



800. Mirrors. It is possible to construct surfaces of metal 

 or silvered glass which are sufficiently smooth to reflect nearly all 

 of the light in accordance with the law of regular reflection ( 794). 

 Such a surface is called a mirror. It may be plane or curved (con- 

 cave or convex). 



If the mirror surface is plane, it follows from the law that the 

 rays will have the same angular relation to one another after 



