LIGHT. 



59 



the angle between them is called the refracting angle. Triangular 

 glass prisms are generally used. 



In Fig. 19, ABC is a section of a prism. A is called the summit or 

 apex, and BC the base. A ray of light, SI, falling upon the prism 

 will not pass through it in a straight line, SIS', but is bent out of its 

 course twice, in accordance with the law of refraction, first from 

 I to I', and then to K. The amount of bending depends on the 

 angle of the prism, its material, and the angle of incidence of the 

 ray, shown in i, while r is the angle of refraction. The angle D 

 represents the angle of deviation. 



Dispersion. In Fig. 19, the ray is represented by a single line 

 throughout. In reality, matters are more complicated, as white 

 light is made up of rays of different colors, each of which has a dif- 

 ferent angle of refraction. The result is that when a beam of white 

 light falls on a prism it does not come through as white light, but 

 the constituent colors are refracted at different angles, giving rise to a 

 band of light containing all the colors of the rainbow, viz., red, 

 orange, yellow, green, blue, indigo, violet ; red being less refracted, 

 violet most. Such a band of colors is known as the prismatic 

 spectrum. 



In Fig. 20, A represents a ray of light which if unbent would strike 

 a screen at X, but the prism P intervening the ray is refracted and 

 at the same time resolved into its constituents, which form the spec- 

 trum, the colors of which are indicated by the initial letters. This 



FIG. 20. 



Prismatic spectrum. 



spreading out of light, and its separation into different colors, are 

 called dispersion. 



The spectroscope is an instrument that serves for conveniently 

 observing the spectrum. Differently constructed instruments are 



