This is to be expected from our Eq. (31.20),because the imaginary part of the denominator, iγkω , is muchsmaller than the term (ω2k−ω2). But if the light frequency ω is very close to ωkthen the resonance term (ω2k−ω2)can become small compared with iγkω and the index becomesalmost completely imaginary, as shown in Fig. 31–5(b).The absorption of the light becomes the dominant effect. It is just this effectthat gives the dark lines in the spectrum of light which we receive from thesun. The light from the solar surface has passed through the sun’s atmosphere(as well as the earth’s), and the light has been strongly absorbed at theresonant frequencies of the atoms in the solar atmosphere.
通常，例如在玻璃中，光的吸收，非常小。这一点，从方程（31.20）中。可以期待，因为，分母的虚部，即iγkω，比项(ω2k−ω2)，要小很多。但是，如果光的频率ω，非常接近ωk，那么，共振项(ω2k−ω2)，与iγkω相比，就会变小，指数就会变得几乎完全是虚的，如图31-5（b）所示。光的吸收，就占了统治地位。正是这种效果，使得我们从太阳接收到的光的光谱中，有黑线。从太阳表面来的光，已经通过了太阳的大气层（正如地球的大气层一样），太阳大气中的原子，有共振频率，光已经被此频率，强烈地吸收过了。
通常，例如在玻璃中，光的吸收，非常小。这一点，从方程（31.20）中。可以期待，因为，分母的虚部，即iγkω，比项(ω2k−ω2)，要小很多。但是，如果光的频率ω，非常接近ωk，那么，共振项(ω2k−ω2)，与iγkω相比，就会变小，指数就会变得几乎完全是虚的，如图31-5（b）所示。光的吸收，就占了统治地位。正是这种效果，使得我们从太阳接收到的光的光谱中，有黑线。从太阳表面来的光，已经通过了太阳的大气层（正如地球的大气层一样），太阳大气中的原子，有共振频率，光已经被此频率，强烈地吸收过了。
, the time average of the square ofthe electric field in the wave. The decrease in E due to absorptionmust mean a loss of energy, which would go into some friction of the electronsand, we might guess, would end up as heat in the material.
