52 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



vibrations correspond to an electric and a magnetic vector, each at right 

 angles to the direction of propagation and mutually perpendicular, 

 varying in time and space according to a sine law. The "direction of 

 vibration" of the light corresponds to the direction of the electric vector 

 and it follows, therefore, that light is affected by passage through a 

 transparent body largely in terms of the electrical state of the matter of 

 which the body is composed. 



To understand this most clearly it is well to consider the theory which 

 was first put forward by Bragg (23) to explain quantitatively the refrac- 

 tive indices of certain crystals. When light falls on an atom, the electrons 

 in the atom suffer a displacement in view of the electric component of 

 the vibration. The atom is said to be polarized,* since the positive 

 nucleus and the cloud of negative electrons suffer a mutual displacement. 

 When two atoms lie close together, as they do when they are linked 

 chemically, then the polarization of one atom has an effect on the 

 polarization of the other. This means that the interaction of matter and 

 light depends in the first instance on the type of chemical bonding 

 between the atoms of which the matter is composed. The mutual effect 

 of one atom on another falls off very rapidly with distance and there- 

 fore in molecules, and in crystals composed of molecules, the total 

 polarization is almost entirely confined to the mutual effects of chemi- 

 cally linked atoms. Now the refractive index n varies with polarization 

 a according to the relation {n^—\)l{n^-\-2)=Ka, where A" is a constant; 

 hence the higher the polarization the greater the refractive index. With 

 rays of light, therefore, taken straight from an incandescent lamp for 

 instance, the total effect of the passage through a crystal will arise as a 

 sort of average effect of all the various types of bonding in the crystal 

 structure. This is because in such rays of light the vibration (electric 

 vector) lies, not in a single plane, but in any and all directions perpendi- 

 cular to the direction of propagation. 



Suppose, now, that the light is first passed through a Nicol prism, or 

 one of its successors or through a sheet of Polaroid. Then the light 

 issuing from any of these bodies is vibrating in one plane only; it is 

 said to be plane polarized. Now the immediate phenomena associated 

 with the passage of such a ray of light through a crystal is obviously 

 more complicated; it will depend not only on the direction of propaga- 

 tion but also, for any one direction of propagation, on the direction of 

 vibration. Consider, for instance, the case of a diatomic molecule 



* The word polarized is unfortunately used in two senses. Polarization of 

 atoms refers to the displacement of positive and negative parts. Polarized as 

 applied to light refers to something quite difierent; this will be referred to 

 later. 



