THE OPTICS OF PHOTOGRAPHIC LENSES 



11 



in vacuum to its velocity in a transparent material is called the "refractive index" 

 of the material (see page 12). In practice, however, the refractive index is taken as 

 being the ratio of the velocity of light in air to that in the material, since practically 

 all lenses are used in air and the refractive index of the material relative to air is the 

 really significant figure. The refractive index of air relative to vacuum is about 

 1.00028. Some other typical refractive indices are given in Table I. 



Since light consists of waves of some kind, there must be a wavelength (X), which 

 is the distance from crest to crest measured along the direction in which the light is 

 traveling; and there must be a definite frequency {v) or number of waves passing a 

 given point in a second. Furthermore, if c is the velocity of light, then these quan- 

 tities are related by 



c = X^ (1) 



It is found that the velocity c is about 3 X lO^" cm. per sec, or 186,000 miles per sec. 

 for light of all colors in air, but it is also found that light of any one pure spectral 

 color has a definite frequency p and hence a definite wavelength X in air. For light 

 which is visible to the eye, these frequencies are very high, and the wavelengths are 

 very short. Light waves too short to be seen are called ultraviolet and will affect a 

 photographic emulsion or a photoelectric cell; light waves too long to be visible are 

 called infrared, of which the shorter infrared waves up to X = 0.0012 mm. can be 

 photographed by means of special infrared-sensitive emulsions. 



In Table II are given the approximate limits of the regions in the spectrum which 

 appear to have the colors stated, but it should be remembered that color is a physio- 

 logical or even a psychological phenomenon and that the colors of natural objects are 

 never pure spectral colors but always more or less broad bands or mixtures of various 

 pure colors. White light consists of a mixture of all the colors of the spectrum. 



Table II. — Approximate Wavelength and Frequency Limits of Colors 

 IN THE Visible Spectrum 



Color in the spectrum 



(Infrared) . . . 



Red 



Orange 



Yellow 



Green 



Blue 



Violet 



(Ultraviolet) 



Frequency limits, 

 per sec. 



Below 4.0 X 101 

 4.0-4.8 

 4.8-5.0 

 5.0-5.2 

 5.2-5.9 

 5.9-6.5 

 6.5-7.5 

 Over 7.5 



Wavelength limits 

 (in air), microns 



Greater than 0.75 

 0.75-0.63 

 0.63-0.60 

 0.60-0.58 

 . 58-0 . 51 

 0.51-0.46 

 0.46-0.40 

 Below 0.40 



In the above table, the wavelength limits are given in microns. The micron 

 (written n) is equal to one-thousandth of a millimeter. Wavelengths are often 

 expressed in terms of angstrom units (lA = 10~^ ju = 10"'^ mm.) or sometimes in 

 millimicrons (m/x). For example, the wavelength of monochromatic sodium light 

 is 5893 A. or 589.3 m/x or 0.5893 m or 0.0005893 mm. This length is approximately 

 1/50,000 in. 



Since the velocity of light is less in glass than in air, it follows that the light waves 

 will become closer together in glass, as indicated schematically in Fig. 2, and hence 

 the wavelength is reduced in glass to the same extent as the velocity. On emerging 

 into air again, both the velocity and the wavelength resume their original values. 



