Calculating the imaginary part of the index of refraction of air involves understanding that it is primarily non-zero near the absorption frequencies of air molecules. These absorption frequencies are found in the infrared and ultraviolet ranges, making them negligible in the visible spectrum. Absorption coefficients for these ranges are available in tabulated form, which can aid in the calculation. Therefore, the focus should be on these specific frequencies when determining the imaginary component. This understanding is crucial for accurate optical modeling in various applications.