Question

What type of gas would create an absorption spectrum? [pick best answer]

		A cool gas in front of an even lower temperature gas
		A high temperature gas in front of a low temperature gas
		A gas which contains at least 50% hydrogen
		Any type of gas can create absorption spectra
		A cool gas in front of a higher temperature gas

Answer 1

To get absorption spectrum we need to have a continuous e-m spectrum.

In absorption, a low energy atom gets excited to high energy state. The lower orbit electron absorbs certain line of spectra and jumps to higher level orbits.

To get absorption spectrum we need a low energy gas (or relatively low temperature).

In theory, any gas would show absorption spectra but a higher energy gas would more tend to show emission spectra more than absorption spectra.

So what do we need

1. A relatively cool gas

2. A high energy background to provide continuous spectrum (like a star)

None of the options are idle for our purpose but assuming higher temperature gas can provide a continuous spectrum, the best answer is

A cool gas in front of a higher temperature gas.