Why Do Absorption and Emission Lines Appear?

How does source–gas–observer geometry change the measured spectrum?

Rearrange a bright source, wavelength-selective gas, and an observer to understand dark dips and bright peaks.

White starlight crosses a translucent cool gas cloud before reaching a spectrograph, leaving dark absorption lines in a rainbow. — Vastward artistic reconstruction. The geometry is conceptual; the spectrum and gas cloud are not drawn to scale.

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