xi LIGHT 181 



(2) If a circle be described about the point of incidence and per- 

 pendiculars be dropped upon the normal, from the intersections of 

 this circle with the incident and refracted rays, the ratio of the 

 lengths of these perpendiculars is constant for any two given 

 media. 



Refraction by a Prism. The deviation of a beam of light caused 

 by the action of a prism depends upon (a) the angle of the prism, 

 (6) its material, (c) the thickness of it traversed, (d) the length of 

 the ether- waves. 



Refraction through a Lens. Most lenses are of glass with curved 

 surfaces which are portions of spheres. They can be divided into 

 convex or converging and concave or diverging lenses. The point 

 to which parallel rays after refraction converge, or appear to diverge, 

 is called the principal focus. 



Analysis of Light by a Prism. The points to be remembered in 

 connection with this are : 



(1) That if a beam of monochromatic light, that is, light of one 

 wave length, traverses a prism, it is bent out of its original direction, 

 but the amount of bending produced by any particular prism depends 

 upon the colour of the beam used, being greatest for violet light and 

 least for red light. 



(2) That if light from any source pass through a prism, it is broken 

 up or analysed into its different components, each ray of elementary 

 colour that enters into the composition of the light being bent by a 

 different amount. 



(3) That if a beam of white light from a slit pass through a prism, 

 it emerges as a coloured strip, termed a spectrum. 



A beam of white light, /When 



it traverses 



/ a prism is \ Red 

 broken up into Orange 



Yellow 

 Green 

 Blue 

 Indigo 

 Violet 



White light can, therefore, like a chemical compound, be broken 

 up into simpler elements. 



Recpmposition of Light The Colour Disc. The analysis of white 

 light is noted above ; the synthesis, or recomposition, can be 

 effected 



(1) By making the coloured band or spectrum, produced when 

 light has passed through one prism, traverse a second prism having 

 its refracting angle reversed. 



(2) By rotating a disc of cardboard painted in segments of violet, 

 blue, green, yellow, orange, and red. 



