This lecture covers the derivation of the resolution power formula for optical systems, which depends on the order of fringes, the number of lines in the grating, and the smallest perceivable wavelength difference. By applying the formula, a numerical value of 0.2 nanometers is obtained, indicating the ability to distinguish light within a small range around the nominal wavelength of 400 nanometers. The process of selecting the correct answer from multiple choices is also discussed.