(Brief) Introduction to Evanescent Waves

• Two transparent media with different refractive indices (n₁ and n₂) contact at an interface. The absorbing species reside in the 2^nd medium.
  
• A light beam reflects at an interface at an angle that exceeds the critical angle. The evanescent field extends beyond the interface into the 2^nd medium to a depth, d_e.
  
• Optical absorption by species entrained in the 2^nd medium is derived from the ratio of the initial and reflected light intensities.
  
• d_e, the "effective thickness" of this evanescent wave corresponds to the equivalent thickness required to obtain the same spectral contrast of a transmission experiment. The "effective thickness" leads to a Beer-Lambert-like law for evanescent wave spectroscopy.