Final answer:
To clearly observe the rotational contribution in an absorption spectrum, a high resolution is necessary due to the finely spaced rotational levels. Instruments must have the capability to differentiate these small differences in frequency within the molecular spectra for precise measurement.
Step-by-step explanation:
To clearly see the rotational contribution in an absorption spectrum, a high resolution is required. This is due to the very fine structure of the rotational levels within a vibrational band. Instruments need to have sufficient resolving power to differentiate between the closely spaced absorption peaks that represent the rotational transitions. In the context of molecular spectra, the resolution required to see details such as the rotational contribution is typically higher than for just observing the gross structure of the spectrum, such as the vibrational bands alone.
For example, with HCl and an estimated separation between absorption peaks of Δf ≈ 5.5 × 10¹¹ Hz, a high degree of resolution is necessary to distinguish these peaks and accurately measure the spectrum.
Considering diffraction limits and instrument quality, such as those referenced for telescopes like Arecibo and for microscopes, we understand that resolution plays a critical role in the clear observation of fine details. Factors such as the size of the objective lens in a telescope or the wavelength of light used in a microscope directly impact resolving power.