To determine the speed of light through a glass plate, the index of refraction is essential, as it relates the speed of light in the medium to that in a vacuum. The formula V = C/N is used, where V is the speed of light in the glass, C is the speed of light in a vacuum (approximately 3x10^8 m/s), and N is the index of refraction of the glass. The discussion clarifies that the goal is to measure the speed of light in the glass rather than in a vacuum. Accurate values for the index of refraction are necessary for precise calculations. Understanding these relationships is crucial for experimental physics involving light propagation in different media.