58 



CHEMICAL PHYSICS. 



Upon this reflection depends the formation of the image seen in mirrors. 

 If a reflecting surface were an absolutely smooth plane, it would be invisible, 

 and we would see in it simply the images of other subjects. Most objects are 

 not bounded by absolutely smooth surfaces, and, consequently, the light which 

 falls upon them is scattered or diffused, thus rendering them visible in all 

 directions. 



Refraction. While a light ray travels in a straight line through 

 homogeneous media, the ray is bent when passing from a medium 

 of one density to that of another density, as from air to glass or to 

 water. Under these conditions, unless the ray enter perpendicularly, 



it is bent out of its course, still moving, 

 however, in a straight path in the 

 second medium, but in a different 

 direction from that in the first. This 

 bending of rays is known as refrac- 

 tion. It is refraction which causes a 

 straight stick, when held obliquely 

 in clear water, to appear bent at the 

 point of entering the water. 



In Fig. 18, SI represents a ray of 

 light entering a denser medium say 

 a plate of glass with parallel sides. 

 Here the ray is bent toward the per- 

 pendicular N'R ; on leaving the denser medium and re-entering the 

 air it is again bent, but away from the perpendicular to such an 

 extent that the rays SI and US' (or rather their extensions) are 

 parallel to one another. 



Refraction through prisms. A prism, in optics, is any trans- 

 parent medium comprised between two plane surfaces inclined to 



Refraction by a parallel plate. 



Refraction through a prism. 



each other. The intersection of the two planes is called the edge, and 



