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 .

A. appear as a continuous rainbow of colors
B. be an emission spectrum
C. appear completely dark
D. still be an absorption spectrum

B. be an emission spectrum.

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.

Stars are perfect examples for Kirchhoff’s laws. In the case that the stars have surrounding material (diffuse gas), the atoms, molecules or ions in the medium are excited by the radiation that comes from the stellar atmosphere, thus producing an emission spectrum.

The emission spectrum has its origin when a electronic transition occurs. An electron in an atom or ion will absorb energy (photon) coming from a 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.

Key terms:

Electronic transition: When an electron passes from one energy level to another, either for the emission or absorption of a photon.

Ion: An atom electrically charged due the add or loss of electrons.  

Photon: Elementary particle that constitutes light.