Final answer:
A cool, dilute gas between an observer and a hot continuous source will produce absorption lines. These appear as black lines where the gas has absorbed specific wavelengths of light. This phenomenon allows scientists to identify the gas's composition.
Step-by-step explanation:
When a cool, dilute gas is situated between an observer and a continuous source of heat, the gas absorbs specific wavelengths of light, leading to the observation of absorption lines. This absorption spectrum appears as black lines within the continuous spectrum of light because the gas has absorbed those particular wavelengths, leaving dark gaps in the otherwise continuous sequence of colors. The concept of absorption lines is beautifully illustrated by the Fraunhofer lines in the solar spectrum, which correspond to the wavelengths absorbed by the solar atmosphere's gases. Furthermore, each element or compound in the gas phase has a unique pattern of these lines, which acts as a spectral signature, allowing for precise identification.
When the same gas is heated to high temperatures, it emits light at those specific wavelengths, revealing a pattern of emission lines. The pattern of an element's emission lines is identical to its absorption lines, but instead of appearing as black gaps in a continuous spectrum, they appear as bright, distinct wavelengths on a dark background if the emission is viewed without a continuous spectrum in the background. Using this information, scientists can deduce the composition of a gas whether it is observed through absorption or emission.