Principles of Microscopy 



DISPERSION 



Fig. 2. Dispersion and refraction by a glass prism. Refraction is a measure of the 

 amount a beam of light is bent. Dispersion is a measure of the amount by which rays 

 of different colors are bent. 



medium in relation to air. Glass, for example, has about one and one- 

 half times the optical density of air, so that its index of refraction, i in 

 most books, is about 1.5. The value of i for water is about 1.3 and for a 

 diamond about 2.4. 



All these figures have been given as "about" so much because an 

 accurate figure can only be given for a single wavelength, that is, a single 

 color, of light. Each wavelength is bent a different amount so that a 

 pencil of white light passing through a prism, or lens, is dispersed as 

 well as bent. This is also shown in Fig. 2. The difference between the 

 index of refraction for red light, which is bent least, and violet light, 

 which is bent most, is known as the "dispersion." In glass, and most 

 naturally occurring transparent substances, a high index of refraction 

 and a high dispersion go together. For example, a diamond has a 

 value for i of 2.41 for red light and 2.47 for violet light, giving a dis- 

 persion of 0.06. These facts, as will be apparent in a moment, make life 

 very difficult for lens designers. 



Now let us examine the formation of an image by a lens. Figure 3 

 shows a simple lens forming an image of an object. This object is an arrow 

 seen in front view at the far left of the picture, and in side view just to the 

 left of the lens. Rays diverging from the lower part of the object to the 

 lower part of the lens strike the lens at a relatively small angle and are 

 therefore bent relatively little. It follows that they go a relatively long 

 way before converging again to form the bottom part of the image. Rays 



