pic taken from:http://snews.bnl.gov/popsci/spectroscope.html
Emission takes place in the dark otherwise you cant see the emitted photons. A spectrometer will diffract it to different angles to distinguish the wavelength emitted showing lines
Absorption by gas takes place if light of continuous freq are available I.e white light.The missing lines are due to photons taken away by the gas .
The lines in emission and absorption correspond because the energy levels involved in the two processes are the same.
