XI 



LIGHT 



175 



noticed that the light is always bent or refracted towards the 

 thick part of the prism. 



The course of a ray of light through a prism can be found by 

 a simple experiment. 



EXPT. 172. Stand a prism upright, that is, upon one of its 

 ends, upon a piece of white paper. Stick two pins into the 

 paper in the positions D E (Fig. 87), place two more E' D' on the 

 opposite side of the prism so that the four appear in a straight 

 line when looking through the prism. Draw the outline of the 

 prism a&c, and then take away the prism and the pins and 

 connect the pinholes as shown in the diagram. It will be found 

 that the ray is bent towards the base of the prism both when it 

 enters and emerges. 



Eefraction through a Lens. Most lenses are of glass with 

 curved surfaces which are portions of spheres. In some lenses 

 one surface is quite plane. All lenses 

 can be divided into two classes convex 

 or converging, and concave or diverging. 

 Converging lenses can be told at once by 

 their power of forming an image of a 

 distant object like the sun, or by that of 

 magnifying. Concave lenses form no 

 image in this way, and, moreover, in- 

 stead of magnifying they make objects 

 appear smaller when viewed through 

 them. 



To understand their action upon the 

 course of rays of light through them it is 

 simplest to regard them as being built up 

 of parts of prisms in contact, as shown in 

 Fig 88, where a convex lens is built up in 

 this way. A ray of light falling upon any 

 one of these prisms is refracted towards its thicker part, and con- 

 sequently they all converge towards a point, which, if the incident 

 rays are parallel, is known as the principal focus of the lens, as 

 F in Fig. 89. To actually find the distance of the principal focus 

 away from the centre of the lens, that is, its focal length, it is 

 only necessary to form an image of the sun by it on a screen and 

 to measure the distance between the lens and the screen. 



FIG. 88. Lens built up of 

 parts of Prisms. 



