Bonds are springs, not rigid sticks
We often draw molecules as balls joined by stiff sticks, but a real chemical bond behaves much more like a spring. The two atoms it links are forever wobbling: the bond stretches a little longer, then squeezes a little shorter, over and over, billions of times a second. A heavy atom on a stiff bond wobbles slowly and gently; a light atom on a weak bond wobbles fast. This restless springiness is what vibrational spectroscopy listens to.
Just like the electrons we met before, this wobbling is quantized: a bond can vibrate only with certain allowed energies, rungs on a molecular energy level ladder. The spacing of these vibrational rungs happens to match the energy of infrared light — the warm, invisible band just past red. So to make a bond jump to a stronger vibration, you feed it an infrared photon of exactly the right energy.
Infrared spectroscopy: a molecule's fingerprint
In infrared spectroscopy, you pass infrared light through a sample and record which infrared colours get absorbed by all the vibrating bonds. Because each type of bond — a carbon–oxygen double bond, an oxygen–hydrogen bond, a carbon–hydrogen bond — vibrates at its own characteristic energy, each shows up as an absorption at its own predictable place. A chemist glancing at an infrared spectrum can often say 'there is an O–H here, a C=O there' the way you recognise instruments in a song.
Normal modes: a molecule's set ways of moving
A molecule with several atoms can wobble in many ways at once, and it could look hopelessly messy. The saving idea is that any complicated jiggle can be broken into a small set of simple, repeating motions called the normal modes of vibration. In each normal mode, all the atoms move in step at a single shared frequency — they all reach their extremes at the same instant. A water molecule, for instance, has exactly three: a symmetric stretch (both O–H bonds lengthen together), an asymmetric stretch (one lengthens as the other shortens), and a bend (the H–O–H angle opens and closes like scissors).
Each normal mode has its own energy and so its own spectral line in the infrared. This is why the number and positions of infrared peaks tell you so much about a molecule's shape: count the modes, see their energies, and you are reading the molecule's mechanical blueprint.
Raman: the clever cousin
There is a second way to watch bonds vibrate, called Raman spectroscopy. Instead of absorbing infrared, you shine a single bright colour of light — a laser — at the sample and watch the light that *scatters* off. Almost all of it bounces back unchanged, but a tiny fraction limps away with slightly less (or slightly more) energy, having donated a little to set a bond vibrating, or stolen a little from one already vibrating. The size of that energy change reveals the very same vibrations.
Why bother with a second method? Because infrared and Raman are wonderfully complementary: some vibrations show up strongly in one and faintly or not at all in the other. A symmetric stretch in a molecule like carbon dioxide, for instance, is invisible in infrared but bright in Raman. Using both gives a fuller picture. Raman also works happily through glass and even through water, which makes it a favourite for examining gemstones, artworks, and living tissue without touching them.