280 DISPERSION AND DIFFRACTION [Ch. IX 



shortening of the path is also very strikingly illustrated by the cover- 

 glass (fig. 31-32, § 80-81). 



§ 452. Dispersion. — This is the separation of waves of composite 

 light into groups which appear of different colors to the normal eye. 

 If white light is dispersed there results the familiar rainbow, or 

 spectrum. 



As this dispersion is due to the different refrangibility of the different 

 wave lengths, the shortest waves being most bent, one would expect 

 that the amount of bending would be in exact proportion to the wave 

 lengths. This is true if one uses a diffraction grating and forms a 

 normal spectrum (fig. 146). When a prism is used to produce the 

 dispersion (145, 2), the dispersion is not in exact relation to the wave 

 length. In general the red end of the spectrum is condensed and the 

 blue end expanded (fig. 147, 148-149). Different kinds of glass dis- 

 perse differently and the same is true of transparent minerals, quartz, 

 fluorite, etc. This makes achromatism possible. As pointed out by 

 Newton, if the dispersion were in exact proportion to the wave length 

 as with gratings, whenever dispersion is overcome, refraction would 

 also be overcome and no achromatic combination of lenses would be 

 possible. 



§ 453. Diffiraction. — This is the bending of light past the edge of 

 objects. Instead of the light all going in a straight line beyond an 

 object, especially a narrow strip, some of it extends as if split off 

 from the main beam at the edge of the obstruction. These dif- 

 fracted beams may give rise to independent or so-called spurious 

 images. With low powers the diffracted light does not cause com- 

 plications, but with high powers the diffraction fringes and diffrac- 

 tion disc may produce effects very difficult of interpretation. (See 

 § 474 where there is a discussion of the part played by diffracted 

 light in microscopic images.) 



Lenses and Images 



§ 454. Lenses. — A lens is a transparent body having one or both 



of its opposite sides curved. The curves are most frequently spherical, 



and may be either convex or concave. If both the surfaces are curved 



the lens may be considered as composed of segments of two spheres. 



