Vibrating bonds absorb infrared energy at a frequency that corresponds to the vibrational frequency of the bond.
In organic chemistry, this corresponds to frequencies of 15 to 120 THz.
These frequencies are expressed as wavenumbers:
#"wavenumber" = "frequency"/"speed of light" = ν/c#
So the wavenumbers range from 500 to 4000 cm⁻¹.
Molecules do not have rigid bond lengths and bond angles.
Rather, bond lengths and angles represent the average positions about which atoms vibrate.
Each vibration may cause a change in the dipole moment of the molecule. The alternating electrical field of the infrared radiation can interact with the alternating dipole moment.
If the frequency of the radiation matches the vibrational frequency, the bond will absorb the radiation. The amplitude of the vibration will increase.
The spectrometer detects this absorption and records it as a peak in a plot of transmission versus frequency.
Within a narrow range, each type of bond vibrates at a characteristic wavenumber. This makes infrared spectroscopy useful to identify functional groups in a molecule.
Here’s a short table of common absorption frequencies.
Notice how you can identify the important vibrations in the spectrum of ethyl acetate. The C=O and C-O absorptions tell you that this is an ester.
Here’s a video that gives a simple explanation of infrared spectroscopy.