Some black holes show a distinct iron emission line.

有些黑洞会很明显有铁的发射光谱。

But what if there is no bright iron emission line?

可是，如果黑洞不发出明显的发射线怎么办呢？

And neon signs give off the particular emission lines of this noble gas.

霓虹灯发出这种稀有气体的特殊射线。

Fluorescent light bulbs appear white but reveal specific emission lines instead of a continuous spectrum.

荧光灯泡看起来是白色的，但显示的是特定的射线，而不是连续的光谱。

The water emission lines were bright.

水发射线很亮。