88 



MICROSCOPY 



as vibratory in nature, and its movement can be pictured as a wave (as 

 shown in Fig. 8-4). The wavelength is the distance from any point on 

 one wave to the same jwint on the next wave, as indicated in the dia- 

 gram. Another feature of vibratory energy is a frequency, which is the 



Fig. 8-4. Wave representation of radiant energy. In curve A, the wave- 

 length (a) is twice as great as in curve B; in curve B the frequency is 

 twice as great as in curve A. 



number of vibrations in a unit of time. In the diagram, curve A has a 

 low frequency, while curve B has a higher frequency. The speed of light 

 (c) is constant (3 X 10^*^ cm/sec in a vacuum), and the frequency and 

 the wavelength multiplied by each other must equal this constant. In 

 other words, the speed of light becomes a proportionality constant, and 

 we arrive at this equation : 



X ^ - or 



V 



Xv = 



Lambda (X) is the symbol commonly used for wavelength in centi- 

 meters or in fractions or multiples of centimeters. Frequency is given 

 the symbol nu (i^), and is expressed in reciprocal seconds (sec~0, or 

 what amounts to vibrations per second. Another useful way of describ- 

 ing the frequency is in terms of a wave number, that is, the number of 

 vibrations in a certain unit of length. In the electromagnetic spectrum 

 we find a continuous variation in wavelength and frequency from one 

 end of the spectrum to the other. At one extreme, wavelengths are very 

 long, and frequencies are very low. At the opposite end, wavelengths 



