A radiation beam is scatteredby a particle in its path when its direction of propagation is altered without absorption taking place. Scattering may take place by reflection, refraction, or diffraction of the radiation beam (Figure ). We define the scattering efficiency of a particle as the probability that a photon incident on the particle will be scattered. Figure 4 shows the scattering efficiency of green light (l = 0.5 mm) by a spherical water particle as a function of particle size. Scattering is maximum for a particle radius corresponding to the wavelength of radiation. Larger particles also scatter radiation efficiently, while smaller particles are inefficient scatterers. Atmospheric aerosols in the accumulation mode are efficient scatterers of solar radiation because their size is of the same order as the wavelength of radiation; in contrast, gases are not efficient scatterers because they are too small. Some aerosol
particles, such as soot, also absorbradiation.
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