MICROSCOPY . i . f : 89 



are very short, and frequencies are high. The visible region of, the spec- 

 trum hes near the middle of the much broader spectrum. There is noth- 

 ing unique about visible light; it merely corresponds to the ability of 

 our eyes to perceive light. The red end of the visible spectrum' consists 

 of the longer wavelengths and lower frequencies, while the blue-violet 

 end has shorter wavelengths and higher frequencies. Immediately be- 

 yond the visible spectrum and continuous with it are the infrared and 

 the ultraviolet. It is difficult to draw a line between the red, visible light 

 and the infrared, but usually this line falls around a wavelength of 700 

 millimicrons (m/w.). The average human eye can see wavelengths as short 

 as about 400 mfi; anything below this limit lies in the ultraviolet. 



Refraction: When a light beam passes from one medium into another 

 in which it travels more rapidly or more slowly, the direction of move- 

 ment is changed. The moving beam can be imagined as a progression 

 of "wavefronts" moving outward from a distant source, each front one 

 wavelength ahead of the next. Near the source these fronts will be 

 spheres, but, as the distance from the source increases, any small segment 

 of the sphere behaves as a plane. If such a beam of light, moving through 

 air, approaches the surface of a piece of glass— through which it moves 

 more slowly— either of two events might occur. The beam might be 

 reflected, with an angle of reflection equal to the angle of incidence, or 

 the light beam might enter the glass. However, since it moves more 

 slowly in the glass, any oblique beam will change direction sHghdy upon 

 entering the glass. The angle of bending depends upon the ratio of the 

 speeds in the two media. Figure 8-5 shows a train of wavefronts approach- 

 ing an air-glass surface. The angle of reflection is equal to the angle of 

 incidence, but the direction through the glass is different. This bending, 

 resulting from the entry into a different medium, is called refraction. 



The angles 6 and 6', which the wavefronts make with the surface, 

 depend upon the speed of the light in the two media according to the 

 relationship 



sm 



9 Vi W2 



sin 9' V2 fii 



where vi and V2 are the speeds in the two media, and n2 and wi are the 

 respective refractive indexes. 



The refractive index is a ratio comparing the speed of light in any 

 medium with that in a vacuum. Because the refractive index of air is 

 only very slightly greater than unity, for practical purposes, glass and 

 other materials are commonlv referred to air as a standard. 



