264 



HANDBOOK OF PHOTOGRAPHY 



In passing out of the denser transparent medium into free space again, the velocity 

 of propagation will be increased. Consequently, the light wave will again be refracted 

 but in a direction opposite to that of the first refraction. 



Ordinarily natural light vibrates in all possible directions perpendicular to its 

 direction of propagation. However, it is possible to restrict to a single plane the 

 transverse displacement of the wave, and light vibrating in this manner is plane 

 polarized. 



Like sound or water waves, light waves curl around obstacles in their path, but 

 because of the tremendous size of any physical object which we can observe compared 

 to the wavelength of light, this effect is so very slight as to be practically nonexistent. 

 Therefore we customarily regard light as being rectilinearly propagated, i.e., as travel- 

 ing in straight Lines. 



Visibilii-y curve at 

 very low light levels, 



I.O 



0.8 



41 mm 



Visibilify curve for 

 ( norma/ lighi levels 



'm 0.6 

 > 



0.4 



0.2 



350 400 450 500 550 600 650 700 750 

 Wavelength in Millimicrons 



Violet I Blue iGreenlYellowlcPn^'jRed 



Fig. 4. — Relative visibility of the normal human eye at normal and greatly reduced light 

 levels, showing the Purkinje shift. 



According to the quantum theory, electromagnetic radiation (of which light is an 

 example and forms a small portion of the total range of possible and available electro- 

 magnetic radiation) is always emitted and absorbed in discrete quantities. The unit 

 of radiation is the quantum or photon. When one of these quanta is absorbed, the 

 energy of the quanta manifests itself as radiant energy or waves which may have their 

 wavelength in the visible region so as to produce light. The subject of quantum 

 theory is beyond the scope of this book and is not essential for an elementary under- 

 standing of photographic principles. 



The eye is not uniformly sensitive to all wavelengths (colors) within the visible 

 spectrum, which extends from about 400 to 700 m/x. For a given amount of energy 

 (light intensity) at the red end of the spectrum, the eye is relatively insensitive. The 

 sensitivity of the eye increases as the wavelength is shifted from the red, through the 

 red-orange, orange, orange-yellow, yellow, and for moderate light intensities finally 

 reaches a maximum for the average human eye in the yellow-green region. Beyond 

 this point of maximum sensitivity, the sensitivity of the eye decreases as we progress 

 through the green, blue-green, blue, blue-violet, and trails off in the violet region. It 

 is safe to say that probably no two human eyes are exactly alike in their sensitivity or 



