What is meant by dispersive power?

Dispersive power refers to the ability of a medium to separate light into its constituent colors when it passes through it. This phenomenon occurs due to the varying degrees of refraction experienced by different wavelengths of light. For instance, in a prism, shorter wavelengths (like blue and violet) are refracted more than longer wavelengths (like red and orange), effectively spreading the light into a spectrum. This characteristic is fundamental in applications such as spectroscopy, where the separation of light into its colors allows for analysis of the spectral composition of light emitted or absorbed by materials.

The other options relate to different properties of light but do not define dispersive power. Energy in light emitted by a source pertains to the intensity and properties of the source itself rather than the medium's ability to disperse light. The speed of light in a medium describes how quickly light travels through that medium, while the distance light travels in a vacuum relates to its propagation characteristics in empty space, neither of which is directly tied to the concept of dispersion.