Ixxvi 



INTRODUCTION TO OPTICS. 



spends with that of the lights ; for each light makes the opaque body cast 

 a different shadow, as illustrated by Jig. 5. It represents a ball, A, 

 lighted by three candles, BCD: the light B produces the shadow 6, the 

 light C the shadow c, and the light D the shadow d. Now what becomes 

 of the rays of light which opaque bodies arrest in their course, and the 

 interruption of which is the occasion of shadows ? Thfs leads to a very 

 important property of light, Reflection. 



Fig. 5. 



When rays of light encounter an opaque body, which they cannot tra- 

 verse, part of them are absorbed by it, and part are reflected, and rebound 

 as an elastic ball which is struck against a wall. Light in its reflection 

 is governed by the same laws as solid perfectly elastic bodies. If a 

 ray of light fall perpendicularly on an opaque body, it is reflected back in 

 the same line towards the point whence it proceeded ; if it fall obliquely, 

 it is reflected obliquely, but in the opposite direction, the angle of inci- 

 dence being equal to the angle of reflection.* If the shutters be closed, 

 and a ray of the sun's light admitted through a very small aperture, and 

 reflected by a mirror, on which the ray falls perpendicularly, but one ray 

 is seen, for the ray of incidence and that of reflection are both in the 

 same line, though in opposite directions, and thus are confounded together. 

 The ray, therefore, which appears single, is in fact double, being composed 

 of the incident ray proceeding to the mirror and the reflected ray returning 

 from the mirror. These may be sepa- Fig. 6. 



rated by holding the mirror, M (fig. 6), 

 in such a manner that the incident ray, 

 A B, shall fall obliquely upon it ; then 

 the reflected ray, B C, will go off in 

 another direction. If a line be drawn 

 from the point of incidence, B, perpendi- 

 cularly to the mirror, it will divide the 

 angle of incidence from the angle of re- 

 flection, and these angles will be equal. 



It is by reflected rays only that we see opaque objects, Luminous 



* See Mechanics. 



