Molecules with 100% transmittance do not absorb energy, so only molecules with lower transmittance values produce infrared spectra.

A compound's IR spectrum is the plot of intensity or percentage transmittance versus wavenumber, where percentage transmittance is the ordinate and wavenumber is the abscissa.

Downward signals on a spectrum indicate absorption bands at specific frequencies in wavenumbers. For instance, in decane, peaks around 3000, 1465, and 1375 cm^-1, signify energy absorption at these frequencies.

Signals on the left indicate high energy, high-frequency radiations, whereas those on the right denote low energy, low-frequency radiations.

Absorption bands corresponding to most functional groups lie above 1500 cm^-1 in the diagnostic region.

The region below 1500 cm^-1 is the fingerprint region, where each molecule shows a distinctive pattern. For example, 1-butanol and 2-butanol  have similar functional groups but different absorption bands in this region.