CHAPTER V. 



ON REGULAR AND DIFFUSE REFLECTION. 



From his previous work on the reflecting power of silicates the writer 

 concluded that a surface like the earth or the moon, which is composed of 

 silicates, must be selectively reflecting. From an examination of some of 

 the subsequent discussions of the reflecting power of the moon, it would 

 appear that a matte surface can not be selectively reflecting. In fact, the 

 whole misunderstanding seems to hinge on this point. Since the surfaces 

 of the minerals examined by the writer were, in nearly all cases, plane and 

 highly polished, there was no "diffuse reflection," which is an entirely 

 different question from the one of low, "practically zero," reflection, which 

 the writer found to be a common property of certain minerals, for certain 

 regions of the infra-red spectrum. 



When energy is reflected from a plane smooth surface, it is commonly 

 called "regular" (or, less accurately, "specular") reflection. On the other 

 hand, energy reflected from a rough surface suffers "diffuse" reflection. 

 The rough surface is equivalent to numerous small, plane, reflecting sur- 

 faces, the planes of which lie in all directions. In "diffuse reflection" for 

 each infinitesimal surface, the ordinary laws of reflection are obeyed in 

 full, unless the linear dimensions of the reflecting surface or of the ragosi- - 

 ties or inequalities on it are small compared with the wave-length. How- 

 ever, the unpolished surface as a whole destroys all phase relation between 

 the particles in the reflected wave-front, which is no longer plane, but 

 irregular. (See Wood's Optics, p. 36.) This irregularity decreases as the 

 angle of incidence increases, so that for a given roughness we get regular 

 reflection. The long waves will be reflected first, then the shorter ones. 

 "Smoked glass, which at perpendicular incidence will show no image of 

 a lamp at all, will at nearly grazing incidence give an image of surprising 

 distinctness, which is at first reddish, becoming white as the angle in- 

 creases." (Wood's Optics, p. 37.) 



The amount of energy reflected "regularly" from a plane surface will 

 depend upon the reflecting power of the substance. Now, the reflecting 

 power R of any substance is related to its index of refraction n and its 

 absorption coefficient k by the equation 



For "transparent media," i.e., "electrical non-conductors" (see 



35 



