Inverse Wavelength X 1 (|xm ^

Figure 2.7 Interstellar extinction curve. Plotted as a function of inverse wavelength X-1 are both the selective and total extinction. Note that the true wavelength X is shown on the top horizontal scale.

is indicated by the unit vector f in Figure 2.8. We next define the specific flux Fv, also known as the flux density. This is the monochromatic energy per area per unit time passing through a surface of fixed orientation Z, and is given by

Iv p dll

Here p = n ■ z is the cosine of the angle between the propagation direction and the surface normal (Figure 2.8). Since light travels at the speed c, Iv/c represents the energy density propagating in the direction n. The total energy density per unit frequency at a fixed location is therefore

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