What does infrared spectroscopy tell you?

Infrared (IR) spectra are plots of either absorbance or % transmission vs. frequencies in wavenumbers (#cm^-1#), typically #400~"4000 cm"^-1#.

The peaks you see in IR spectra are basically results of focused, filtered light directed at molecules that is at just the right frequency of choice to try to alter the molecule's dipole moment.

These just-right (resonant) frequencies show up on the IR spectrum, because they correspond to the strongest alterations of the molecule's dipole moment, which leads to drastic alteration of the molecule's motion, and thus the resultant peaks that show up are the strongest peaks.

The IR spectra tell you what types of vibrational modes (motion) the molecule responds with after it absorbs that light, and when you figure out which peaks correspond to which motions, you can figure out what functional groups the molecule has and (almost) what the molecule is.

Note: You probably will have to use this with NMR sometime to be able to identify the molecule if it's unknown.

Molecules vibrate and wiggle all around, doing stuff like this: