316 



WELLS'S NATURAL rniLOSOPHT. 



Fig. 254. Thug, in Fig. 244, let A B C be an angle; then 



a will be the sine of that angle, being drawn from 

 a point in the line A B, perpendicular to the lino 

 B C. Two angles may be compared by means of 

 their sines, but whenever this is done, the lengths 

 of the sides of the angles must be made equal, be- 

 cause the sine varies in length according to the length of the lines formin* 

 the angle. 



The general law of refraction is as follows: — 



^ When a ray of lisjht passes from one medium 



^^at is the i • /. i , ^ . . 



general law of to another, the sme of the an<j:le of mciaence 



refraction? . . ' . i • ^ i i 



IS m a constant ratio to the sine or the angle 

 of refraction. 



The proportion or relation between these sines differs when different media 

 are used ; but for the same medium it is always the same. 



Fig. 255. 



Thus, in Fig. 255, let F E be the surface of 

 some refracting medium, as water, and H R, 

 H' R, rays incident upon it, at diiTerent angles ; 

 the former will be refracted in the direction 

 R I' ; o and b will be the sines of the angle 

 of incidence, and c d the sines of the angle of 

 refraction ; and the quotient arising from di- 

 viding b by c, is the same as that from divid- 

 ing a by d. In the case of air and water, 

 the sine of the angle of incidence in the air 

 will be to the sine of the angle of refraction 

 in water as 4 is to 3 •, in any two other me- 

 dia, a different ratio would be observed with equal constancy. 



The quotient found by dividing the sine of 

 the angle of incidence by the sine of the angle 

 of refraction, is called the index of refraction. 



As different bodies have different refractive powers, they will present dii^ 



ferent indices, but in the same substance it is always constant. Thus, the 



refractive index of water is 1.335, of flint glass, 1.55, of the diamond, 2.487. 



Is lieht ever ^^ surface ever transmits all the light which falls upon it, 



but a portion is always reflected. If, in a dark room, we 



allow a sunbeam to fall on the surface of water, the divisioa 



of the light into a reflected and refracted ray will be clearly perceptible. 



,, , , . When the obliquity of an incident ray passing through a 



Under what cir- , ,. ' •' , , ^, -^^ ^ °. ^ .". . 



cuniBtancfs will denser medmm toward a rarer (as through water into air), is 



*"*,¥ reflection g^^h that the sine of its refracting angle is equal to 90°, it 

 •f light occur ? e o \ 



ceases to pass out, and is reflected from the surface of the 



denser medium back into it again. This constitutes the only known instanco 



of the total reflection of light. The phenomenon may be seen by looking 



What is 

 index of 

 fraction ? 



wholly traas- 

 mitted ? 



