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# Spectroscopy - spectrum, absorption spectrum, emission spectrum, Doppler shift

 The Visible Spectrum:  Typically, superheated solids and liquids will emit light of all colors.  We see it as white light, but if we use a spectroscope or prism to separate the light by different frequencies, we see it as a spectrum like the one to the left. The Absorption Spectrum:  When atoms are present as a gas, we find that they will absorb only certain colors of light.  So if we shine white light through a gas then view it with a spectroscope, we find that there are some colors missing. (the missing colors appear as dark lines) These missing colors are the colors that were absorbed by the gas as the light passed through it.  Different atoms absorb different colors, so the dark lines would be in different places for each type of atom. This technique is used to identify what the outer layers of stars are made of. The Emission Spectrum:  When we excite an atom, we find that it gives off (emits) only certain colors of light. Notice that the position  and colors of the absorption spectrum line up with those of the emission spectrum. This means that the colors of light emitted by an atom are the same colors that can be absorbed by that atom. Red Shift:  As a result of the Doppler effect, if a mass of excited atoms are moving away from us, then the wavelengths will become longer making all the colors shift slightly toward the red side of the spectrum.   We call this the Red Shift.  The faster the object is moving, the more the colors will shift.  We know the universe is expanding because most stars show this red shift. More importantly, more distant stars have more red shift, meaning they are moving away at a higher speed. Blue Shift:  As a result of the Doppler effect, if a mass of excited atoms are moving toward us, then the wavelengths will become shorter making all the colors shift slightly toward the blue side of the spectrum.  We call this the Blue Shift.  The faster the object is  moving, the more the colors will shift.

The spectrums shown above are for a mythical substance, Wirtassium.   The spectrums shown have been modified for display purposes.  In real spectroscopy, the emission lines and the absorption lines are much narrower and fainter.