CHAPTER II 



THE OPTICS OF PHOTOGRAPHIC LENSES 



By R. Kingslake 



The Nature and Properties of Light, — In spite of the enormous amount of theo- 

 retical and experimental work that has been done on light, its real nature still remains 

 a mj^stery. The original corpuscular theory of Newton (1643-1727), in which light 

 was supposed to consist of a hail of small discrete particles, was abandoned in favor 

 of the wave theory of Huygens (1629-1695), Young (1773-1829), and Fresnel (1788- 

 1827) because it did not adequately explain the phenomena of polarization, inter- 

 ference, and diffraction. The physical nature of the light waves postulated by 

 Huygens was hotly debated during the nineteenth century, the matter reaching its 

 climax in Maxwell's electromagnetic theory of light (1873). 

 Even this elaborate and comprehensive theory could not explain 

 certain aspects of the photoelectric effect or of the observed 

 spectral distribution in the radiation from a hot bodj^; an attempt 

 to explain these phenomena led Planck (1900) to develop his 

 quantum theory, which is a very elaborated form of corpuscular 

 theory. At the present time, we have the two incompatible 

 theories of light (electromagnetic waves and discrete quanta) 

 in use together, the physicist choosing to adopt whichever theory 

 best fits his experimental conditions. 



Fortunately, in discussing lens action we need consider only 

 the simple wave theory of light, without even inquiring into 

 the nature of the hypothetical medium through which the waves 

 are propagated. We call this medium the ether, and we adopt 

 the simple Huygenian assumption that the wave front is 

 propagated by means of wavelets which start from every point 

 on a wave front, their common envelope constituting the new 

 wave front. In Fig. 1 the full line W represents a wave front at 

 any given instant, the wavelets starting out from a number of 

 points are shown, together with the new wave front W at a later time. 



The velocity of light (c) is very high, being about 3 X 10^° cm. per sec. in vacuum, 

 but light travels slower in passing through matter. The ratio of the velocity of light 



Table I. — Some Typical Approximate Refractive Indices 



W 

 Fig. 1.— Prog- 

 ress of a wave by 

 means of wavelets. 



Substance 



Water 



Alcohol 



Glycerin 



Carbon disulphide 



Monobromonaphthalene 

 Methylene iodide 



Index 



1.33 

 1.36 

 1.46 

 1.63 

 1.66 

 1.74 



Substance 



Fluorite 



Quartz 



CrowTi glass. . . . 



Flint glass 



Dense flint glass 

 Diamond 



10 



Index 



1.43 



1.54 

 1.46-1.53 

 1 . 53-1 . 65 

 1.65-1.92 



2.42 



