This quantity is closely related to the mean intensity Jv tions:

.e., the average of Iv over all direc-

Consider now AIV, the change of Iv along the direction of propagation (Figure 2.9). Over a small distance As, dust grains can remove radiation from the beam through several processes. One is absorption, in which the radiative energy is transformed to internal motion of the grain

Figure 2.8 Definition of the specific intensity Iv. We imagine light propagating within a cone of solid angle Afi centered on a direction n. The vector n can itself be tilted at some angle 6 relative to z. The latter defines the normal direction to the surface area A^ of interest.

Figure 2.8 Definition of the specific intensity Iv. We imagine light propagating within a cone of solid angle Afi centered on a direction n. The vector n can itself be tilted at some angle 6 relative to z. The latter defines the normal direction to the surface area A^ of interest.

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