462 



PHYSICAL SCIENCES. 



NOTE 189, pp. 138, 153, 156. Reflection and Refraction. Let 

 PC/), fig. 48, be perpendicular to a surface of glass or water A B. When 



Fig. 48. 



a ray of light, passing through the air, 

 falls on this surface in any direction I C, 

 part of it is reflected in the direction 

 C S, and the other part is bent at C, and 

 passes through the glass or water in the 

 direction C R. I C is called the incident 

 ray, and I C P the angle of incidence ; 

 C S is the reflected ray, and P C S the 

 angle of reflection ; C K is the refracted 

 ray, and p C R the angle of refraction. 

 The plane passing through S C and I C 

 is the plane of reflection, and the plane 

 passing through I C and C R is the plane 

 _ of refraction. In ordinary cases, C I, 



P C S, C R, are all in the same plane. 



We see the surface by means of the reflected light, which would otherwise 

 be invisible. Whatever the reflecting surface may be, and however 

 obliquely the light may fall upon it, the angle of reflection is always equal 

 to the angle of incidence. Thus I C, I' C, being rays incident on the sur- 

 face at C, they will be reflected into C S, C S', so that the angle S C P 

 will be equal to the angle I C P, and S' C P equal to I' C P. That is by 

 no means the case with the refracted rays. The incident rays I C, I' C, 

 are bent at C towards the perpendicular, ia the direction C R, C R' ; and 

 the law of refraction is such, that the sine of the angle of incidence has a 

 constant ratio to the sine of the angle of refraction ; that is to say, the 

 number expressing the length of I m, the sine of I C P, divided by the 

 number expressing the length of R n, the sine of R Cp, is the same for all 

 the rays of light that can fall upon the surface of any one substance, and 

 is called its index of refraction. Though the index of refraction be the same 

 for any one substance, it is not the same for all substances. For water it 

 is 1*336 ; for crown-glass it is 1*535 ; for flint-glass, 1*6 ; for diamond, 

 2-487 ; and for chromate of lead it is 3, which substance has a higher re- 

 fractive power than any other known. Light falling perpendicularly on a 

 surface passes through it without being refracted. If the light be now 

 supposed to pass from a dense into a rare medium, as from glass or water 

 into air, then R C, R' C, become the incident rays ; and in this case the 

 refracted rays, C I, C I', are bent from the perpendicular instead of towards 

 it. When the incidence is very oblique, as r C, the light never passes into 

 the air at all, but it is totally reflected in the direction C S, so that the 

 angle p C r is equal to p C r' ; that frequently happens at the second sur- 

 face of glass. When a ray I C falls from air upon a piece of glass A B, it 

 is in general refracted at each surface. At C it is bent towards the per- 

 pendicular, and at R from it, and the ray emerges parallel to I C ; but, 

 when the ray is very oblique to the second surface, it is totally reflected. 

 An object seen by total reflection is nearly as vivid as when seen by direct 

 vision, because no part of the light is refracted. When light falls upon a 

 plate of crown-glass, at an angle of 4 32' counted from the surface, the 

 glass reflects 4 times more light than it transmits. At an angle of 7 1' 

 the reflected light is double of the transmitted ; at an angle of 11 8' the 

 light reflected is equal to that transmitted; at 17 17' the reflected is 



