Answer:
The absorption line spectrum shows what we see when we look at a hot light source (such as a star or light bulb) directly behind a cooler cloud of gas. Suppose instead that we are looking at the gas cloud but the light source is off to the side instead of directly behind it. In that case, the spectrum would be an emission spectrum.
Step-by-step explanation:
Kirchhoff’s laws establish that:
- A solid, liquid or dense incandescent gas emits a continuous spectrum.
- A hot and diffuse gas produces bright spectral lines (emission lines).
- A gas of lower temperature against a source of continuum spectrum, produces dark spectral lines (absorption lines) superposed in the continuum spectrum.
According with Kirchhoff's laws it is get an emission line spectrum¹ in the scenario at which the observer is looking directly at a gas cloud with the light source off to the side from the line of sight.
In this case the atoms, molecules or ions in the medium are excited by the radiation that comes from the light source. That is known as an electronic transition², an electron in the atom or ion will absorb the photon coming from the light source and pass to a higher energy level, the electron, upon returning to its base state will emit a photon or a series of photons.
Keys terms:
¹Spectrum: Decomposition of light in its characteristic colors (wavelengths).
²Electronic transition: When an electron passes from one energy level to another, either for the emission or absorption of a photon.