Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 125

Particles in the size range of 20 or more times the wavelength can be described by using concepts of geometrical optics. The incident beam of light can be thought of as being composed of separate rays that interact with the particle. This interaction can be in the form of reflection, refraction, or diffraction processes (Figure 4‐12).

Reflection is a surface effect in which, without entering the particle, the light ray changes the direction after striking it. Reflection will occur if the depth of particle surface irregularities is small relative to the wavelength of light. Light refraction occurs after light enters the particle. Its speed and direction change because of the change in the optical characteristics (refractive index) of the material. Once light has entered the particle, it can also reflect internally. Diffraction is the bending of light around an object caused by interference effects at the particle surface.

Figure 4‐12 Light scattering from large particles (r/λ ≥1) where geometrical optics apply.

Scattering phenomena have been emphasized here, but it should be noted that particles can also absorb light energy, as do gaseous molecules. When light of a specific wavelength resonates with the molecular electrons contained in the particle, it will cause the electrons to jump from one energy state to another. As stated earlier, only light wavelengths that correspond exactly to the molecular energy levels will be absorbed to impart energy to the molecule. As a result, light is again lost from the incident beam.