 Solution

An optical prism separates a white-light beam because its refractive index depends on wavelength. Snell's law therefore gives different emergence directions for different components. With ordinary visible dispersion, violet is deviated more strongly than red. A narrow entrance slit and a collimated beam make a resolved spectrum; a broad overlapping bundle of rays would wash the colours and lines together.

For prism angle $A$ at minimum deviation, the symmetric ray path givesMeasuring a named line's deviation therefore fixes the material's refractive index at a reproducible wavelength.

The Fraunhofer lines are dark absorption features in the solar spectrum, mapped and labelled by Joseph von Fraunhofer. They provide useful spectral landmarks. Measuring a glass's refractive indices at several such lines characterizes optical dispersion, supports comparison of glasses, and helps design an achromatic doublet that brings selected colours to a common focus. Achromatism at two wavelengths does not remove every residual colour error.

The physical explanation is selective absorption. Radiation from the hotter underlying solar layers passes through material whose atoms and ions absorb strongly at their transition wavelengths. The same chemical species can emit at corresponding wavelengths under suitable excitation, connecting solar absorption with laboratory emission spectra. Some lines in a ground-observed spectrum instead arise in Earth's atmosphere, so not every dark feature identifies solar material.

In modern terms, an atomic transition satisfies $\Delta E=h\nu =hc/\lambda$. This explains the reproducible spectral positions rather than treating the dark lines as gaps introduced by a defective prism. Resolving power, line width and source conditions still govern how accurately a particular feature can be measured.