6 The Microscope 



II \ X SIDE VIEW FRONT VIEW 

 ==^ _ CURVEDJMAGE__D|STO_RTED IMAGE" 



OBJECT SIDE VIEW 1 "^^C ' ■ , _\] " ~TT~ 



RAYS FROM EDGE" \. // \_ y^ 



OF LENS ^// \ / 



Fig. 3. Diagram to show the causes of spherical aberration. Rays passing through 

 the edge of the lens are bent more than those passing through the center. The rays 

 passing through the edge of the lens therefore come to a focus nearer the lens, with 

 the result that the image is curved. As those portions of the image that are nearer 

 are larger, it follows diat the image is distorted. 



diverging from the top part of the object to the top part of the lens 

 strike the lens at a relatively large angle and are therefore bent more than 

 the rays striking at a low angle. It follows that they go a relatively 

 shorter distance before converging to form the upper part of the image. 

 It is obvious that this effect is proportional all over the lens so that the 

 image seen in side view is curved. Actually, since the surface of the 

 lens is part of a sphere, the image is also part of a sphere and this 

 effect is known as "spherical aberration." 



Now, the size of an image is dependent on the relative distance of 

 the image from the lens. Hence spherical aberration produces not only 

 an image that is curved but also an image that is distorted in shape 

 when it is cast on a flat surface. This distortion is shown, exaggerated 

 for the sake of clarity, in the front view of the image seen at the far right 

 of Fig. 3. Spherical aberration is the result of refraction. Dispersion pro- 

 duces "chromatic aberration." 



Figure 4 shows exactly the same setup as Fig. 3 but rearranged to 

 show the cause of chromatic aberration. Pencils of light diverging from 

 the object are dispersed as they go through the lens. The red components 

 are bent least and therefore travel a relatively long distance before con- 



.1... 



SIDE VIEW FRONT VIEW 



IMAGE I IMAGE 2 IMAGE 



7f— -sf 



OBJECT SIDE VIEW m -'*-^:<^. i| 3g 



° BJECT ' RAYS FORMING >**W || | 



BLUE IMAGE (I) ]/~ ***++*Jk II RE0 FRINGE 



RAYS FORMING ill ^u-MJ^ 



RED IMAGE (2) l]| \ •/ 



a %/ 



Fig. 4. Diagram to show the causes of chromatic aberration. Light is dispersed ( see 

 Fig. 2) as well as refracted by the lens. The blue components are bent more than 

 the red components, so the blue image is nearer the lens than the red image. There 

 is, therefore, a red fringe around the image seen in front view. 



