Final answer:
The gas in the jar closest to the viewer is a cold gas, as the observed dark spectral lines in the spectrum indicate absorption of specific wavelengths of light when white light passes through it.
Step-by-step explanation:
When using a spectroscope to observe the light spectrum of two jars filled with gas placed one in front of the other, the appearance of dark spectral lines indicates that the gas closest to you is absorbing some wavelengths of light. If the absorption spectra are observed, this means that white light has passed through a cold gas, and certain wavelengths have been absorbed, hence not seen in the spectrum. The gas absorbs specific colors because it is cooler than the light source, and atoms in the gas have absorbed photons that match the energy difference between their electron energy levels. This energy absorption is what causes the dark lines in the continuous spectrum on the viewing screen. Therefore, the jar closest to you contains a cold gas. This property of gases absorbing light at specific wavelengths is fundamental in the field of spectroscopy and provides critical information about the composition and condition of gases and stars.