Final answer:
The intensity of an IR band describes how much light is absorbed at a specific wavenumber, reflected in how dark or pronounced the band appears on the spectrum, not the wavenumber, width, or relative size to other peaks.
Step-by-step explanation:
The INTENSITY of an IR band describes how much light is absorbed by the sample at a particular frequency or wavenumber. In IR spectroscopy, we observe a plot where the x-axis shows the wavenumber (in cm-1), and the y-axis shows % transmittance. Low % transmittance indicates high light absorption, and the intensity of the IR band reflects this absorption. Therefore, the correct answer is (b) describes how dark the band is on the page, which correlates to how strong the absorption is, thus how much energy is being absorbed by the sample. The intensity, in this case, is visually represented by how dark or pronounced a band appears in the spectrum.
The depth or darkness of the band on the spectrum results in an 'upside down' peak. These peaks, which point downward due to the convention of IR spectroscopy, indicate the areas where specific molecular vibrations absorb IR radiation, decreasing the transmittance. The depth of these bands is a relative measure of the number of molecules that are involved in the vibration that gives rise to that particular IR absorption.