There are three types of spectra: continuous, bright line (emission), and dark line (absorption).  A glowing solid or liquid or high-pressure gas will produce a continuous spectrum but only low-pressure gases will produce the two line spectra.

Continuous Spectra

All wavelengths (and frequencies) of light are present.  In the visible range (400 nm to 700 nm), all the colours are visible.





 

Bright Line Spectra

An excited low-pressure gas produces a bright line spectrum.  The spectrum only consists of lines of particular wavelength.  A bright line spectrum is also called an emission spectrum because the gas emits certain wavelengths (frequencies).

Here is a diagram of the experimental set-up to produce a bright line spectrum.


 







Emission spectrum: a pattern of bright lines produced by a hot gas at low pressure


Dark Line Spectra

A dark line spectrum is created when light from a glowing solid or liquid is passed through an unexcited (cool) gas.  The typically continuous spectrum (from the glowing solid or liquid) is now missing certain wavelengths - these missing wavelengths appear as dark lines.  A dark line spectrum is called an absorption spectrum because the gas absorbs certain wavelengths (frequencies).









Absorption spectrum: a pattern of dark lines produced when light passes through a gas at low pressure


Self-Check

SC 2. The emission and absorption spectra of hydrogen are displayed above.  Compare the position of these lines.  What do you notice?  Does this mean that a gas can only absorb and emit a limited number of unique EMR wavelengths?


Self-Check


Contact your teacher if your answers vary significantly from the answers provided here.

SC 2. Both the emission and absorption spectral lines occur in the same place on the spectrum. This means that a gas can only absorb and emit the same, specific wavelengths of EMR.