LAW OF REFLECTION. 



293 



reflection does not involve the nature of the reflecting body, but 

 only the relation between the position in space of its surface and 

 the direction of the incident light ; the law of refraction, on the 

 other hand, involves the nature of both media, the direction 

 taken by the refracted ray being influenced by each of them, and 

 being dependent on certain differences existing between the power 

 possessed by each of transmitting light, as well as on the relation 

 between the surface of the body on which the light is incident 

 and the direction in which the light falls. 



Expt. 332. To Illustrate the Law of Reflection. Place a flat 

 reflecting surface (such as an ordinary lookingglass, or preferably 

 a polished plate of white metal, such as silver, or a vessel of clean 

 mercury (Expt. 281) upon a table, and arrange a candle (fig. 135) in 

 such a way that the 

 reflected image of 

 the flame can be 

 seen in the bright 

 mirror; it will be 

 found that, no 

 matter how the 

 candle (A) may be 

 placed in reference 

 to the mirrqr (B) 

 in order to see the 

 reflection, the eye 

 must be placed at some point (E), which is determined by the 

 following geometrical considerations. 



1. The line AC (or incident ray), the line CE (reflected ray 

 meeting the eye at E), and the line CD, drawn perpendicular to the 

 surface of the mirror (termed the normal at the point of incidence 

 are all in one plane. 



2. The incident ray and the reflected ray lie on opposite sides of 

 the normal at the point of incidence, and are so situated that each 

 ray makes an equal angle with the normal. Or, in other words, 

 the angle ACD is equal to the angle ECD, which is expressed by 

 saying that the angle of incidence is equal to the angle of reflection. 



This is equally true whatever may be the nature of the reflecting 

 surface. A flat plate of glass will reflect the candle exactly in the 

 same way as a plate of polished silver or a basin of mercury at 

 rest, so far as direction is concerned ; but in each case the brilliancy 

 of the reflected light is different, i.e., a different proportion of the 

 incident light is reflected. With glass, when the light falls nearly 

 perpendicularly, so that the angle of incidence is acute, very little 

 is reflected, the majority being transmitted ; but if the light fall 



Fig. 135. Reflection of Light. 



