168 OPTICAL PRINCIPLES. 



Refraction. But when the rays in their passage 

 impinge or are incident upon and enter a transpa- 

 rent medium or material, of a different density from 

 that which they were at first traversing, their course 

 becomes altered, and the line of their direction broken, 

 whence they are said to be refracted. If the medium 

 upon which the rays impinge be denser than that 

 through which they were at first passing, they will 

 be refracted towards a line perpendicular to the sur- 

 face, or they will be refracted towards the perpen- 

 dicular, as it is expressed. 



Thus, as shown in PL XII. fig. 1, the incident ray 

 i, entering the plate of glass, will be refracted at its 

 surface in the direction a r, towards the line JP, which 

 is perpendicular to the surface. 



The extent to which the rays undergo refraction 

 depends upon the degree of density of the medium, 

 and varies in the case of each individual substance ; 

 but it follows a definite law. If, as in PL XII. fig. 2, 

 a circle be drawn around the point b, at which the ray 

 a is incident, b r representing the refracted ray, the 

 lines s i and t r, drawn at right angles to the perpen- 

 dicular p, will form respectively the sines, as they are 

 called, of the angles s b i and t b r ; si being the sine of 

 the angle of incidence s b i } or the angle formed by the 

 incident ray with the perpendicular, and / r the sine 

 of the angle of refraction t b r, or of that formed by 

 the refracted ray with the perpendicular. These sines, 

 for brevity, are called the sines of incidence and of 

 refraction ; and they bear a constant ratio or propor- 

 tion to each other. Taking the sine of refraction as 

 the unit, or as =1, the value of the sine of incidence 

 represents the refractive index or the refractive power 

 of the medium for a ray entering the medium from a 

 vacuum; or, the refractive power of air being ex- 

 tremely small, the value of the sine of incidence may 

 be considered as representing the refractive power 

 from air into the medium. 



