58 LIGHT ABSORPTION EFFECTS 



have some particular refractive index, and the cell will be visible if sus- 

 pended in a medium of another refractive index. This result may be used 

 in the opposite way. By varying the refractive index of the suspending 

 medium, perhaps by varying the sugar content of the medium, it is possi- 

 ble to find a situation of just the right refractive index so that the various 

 parts of the cell are no longer visible. Thereby, the refractive index of 

 these parts of the cell is determined. Since the refractive index is pro- 

 portional to the concentration of a substance, one can draw inferences 

 about the concentrations of various substances in different parts of cells. 



2. Birefringence 



As hinted at in the introduction to this section, light which is not 

 absorbed is re-emitted in all directions, producing what is called scatter- 

 ing of the light; we mentioned the two extreme cases of total reflection 

 and total transmission. Most cases are intermediate. If electron oscilla- 

 tions are equally possible in all directions, the incident unpolarized light 

 beam will emerge essentially as it entered the suspension — except for the 

 phase shift already mentioned. If, however, the particles or molecules in 

 suspension are not isotropic, i.e., the electrons can oscillate more readily 

 in one direction than in another, an incident unpolarized light beam will 

 be split, because waves oscillating in one direction will have their phases 

 shifted more than those oscillating in another direction. The net result is 

 that so-called anisotropic molecules produce two plane polarized emergent 

 beams. Since these emerge in somewhat different directions, the phe- 

 nomenon is called double refraction or birefringence. 



Clearly, a measurement showing birefringence tells us at once that the 

 material being studied is anisotropic, and since the optical anisotropy is 

 due to anisotropy in the permissible movements of electrons, we can infer 

 something about the structure of the material. 



The chief use of this phenomenon in biology presently is in the inverse 

 fashion. We shine plane polarized light on the substance of unknown 

 structure and see what happens to the plane of polarization. Further, if 

 we line up the molecules of a substance (as can be done in several ways), 

 then the refraction (bending of the light) measured along and at right 

 angles to the direction of alignment will tell us even more about the 

 electronic structure of the individual molecules. 



Birefringent structure may arise in several ways, of which a few will 

 now be briefly presented. 



(a) Intrinsic birefringence. The chemical nature and structure of 

 individual molecules may contribute birefringence if there is anisotropy in 

 the movability of the electrons of the molecules. Chemical bonds them- 

 selves may be highly anisotropic, the C — C bond being a good example. 



