102 A CHAPTER ON LIGHT AND COLOUR. 



(iv.) If a ray be reversed at any point of its path, it will return 

 over exactly the same path as that which it has already traversed. 

 A Ray is here conceived of as the smallest portion of light which 

 can be separately transmitted, reflected and refracted. A group 

 or bundle of rays constitutes a Pencil. 



3. — The first of the above laws is sufficiently obvious, but we 

 shall see that Hght, like sound, can bend round a corner, though 

 not sufficiently, in general, to produce effects readily recognisable. 



The production of Images by Reflection in plane, and even 

 in curved, mirrors is familiar to us also. The image is as far 

 behind a plane mirror as the object is in front of it. Curved 

 mirrors produce images not, in general, equal in size to the object, 

 and may produce curiously contorted images. 



The Refraction or bending of rays of light as they pass from 

 air to water or glass causes a change in the appearance of an object 

 seen through such bodies, unless a second bending takes place at 

 a parallel surface to recompense for the first. The production of 

 images by lenses is a practical application of this bending of light 

 by glass, and in telescopes, microscopes, opera-glasses, and spec- 

 tacles is made of great service to mankind. The position, size, 

 and degree of distortion (if any) of the image, can be determined 

 by repeated calculations from the laws given above. 



Refraction is closely connected with Total Reflection. 

 Contrary to what is usually the case, at angles of incidence beyond 

 a certain limit, light traversing a denser medium and meeting a 

 surface of separation from a less dense, is no longer divided into 

 both reflected and refracted waves, but gives only the former, 

 being, for all angles of incidence exceeding this limit, totally 

 reflected into the medium which it is traversing. 



If light be propagated through a medium which is not homo- 

 geneous, its path will not be a straight line. The light from the 

 stars suffers a continuous bending in passing through the earth's 

 atmosphere. In some tropical regions the sun's heat causes a state 

 of the atmosphere to be set up in which layers of differently-heated^ 

 and therefore differently-refracting, air are superposed in such a way 

 as to totally reflect rays of light which were originally travelling at 

 a small upward inclination. In this way the phenomenon of 

 Mirage is produced. 



