

21 



SOME PROPERTIES OF OPTICAL GLASS. 



When a ray of light, passing through air, falls on a glass 

 surface, it becomes bent on passing into the glass by an 

 amount which depends upon the nature of the glass, the angle 

 of incidence and the colour or wave-length of the light. A 

 ray f ailing exactly perpendicular to the glass surface suffers no 

 deviation ; but all other rays are bent to an extent increas- 

 ing with the obliquity of incidence, and in accordance with the 

 following fundamental law of refraction discovered by Snell : 



The sine of the angle between the incident ray and the 

 perpendicular to the surface is always in a constant ration to 

 the sine of the angle between the refracted ray and the 

 perpendicular for a given glass and colour of light, this 

 constant ratio being called the " index of refraction," and 

 denoted by n or /*. 



A parallel beam of light, that is, a plane wave of light, 

 after refraction, still remains parallel, though moving in a 

 different direction. If, however, the beam of light falls upon 

 a convex surface, the rays become convergent, approximately, 

 to a single point or focus, at which an image of the source of 

 light is formed. 



The phenomenon of refraction, therefore, enables us to 

 make optical systems by which images are formed, and in 

 many cases magnified, resulting in the varieties of telescopes, 

 microscopes, photographic lenses, etc., with which the 

 optician has to deal. It is, therefore, obvious that, in the 

 design of lens systems, an accurate knowledge of the indices 

 of refraction of the glasses used is essential. It is found 

 that for light of different colours the index varies greatly, 

 and therefore the index for a definite colour must be pre- 

 cisely defined. Fraunhofer first realised that this could be 

 done by making use of the monochromatic light which is 

 given off by various luminous vapours, and this method is 

 universally adopted. 



The indices of refraction of a glass are almost always 

 given for the homogeneous red, green and violet radiations 

 from a low-pressure hydrogen vacuum tube, which are styled 

 respectively C, F and G', and for the yellow light from a 

 Bunsen flame containing sodium vapour. Since the sodium 

 flame is the most readily obtainable light which is approxi- 

 mately monochromatic, the " mean " index of refraction of 

 a glass is always stated in terms of this particular radiation 

 and denoted by the symbol n the suffix referring to the 

 designation D of the sodium lines in the spectrum. 



