Final answer:
Spectral lines in a spectroscope can be used to determine the chemical composition of a gas by observing the unique patterns of wavelengths emitted or absorbed by each chemical element.
Step-by-step explanation:
Spectral lines, the colorful lines seen in a spectroscope from glowing gases, can be used to determine the chemical composition of their source because each chemical element or compound produces its own characteristic pattern of spectral lines, which is unique to that element or compound. These lines can be observed when a hot, thin gas emits or absorbs light at specific wavelengths. The precise pattern of wavelengths in the spectral lines can be used to identify which elements are present in a gas.