Figure 3-49Continuous, Absorption Line, and Emission Line Spectra This schematic diagram summarizes how different types of spectra are produced. The prisms are added for conceptual clarity, but in real telescopes diffraction gratings are used to separate the colors. A hot, glowing object emits a continuous spectrum. If this source of light is viewed through a cool gas, dark absorption lines appear in the resulting spectrum. When the same gas is viewed against a cold, dark background, its spectrum consists of just bright emission lines.