Question

What are Fraunhofer lines? What is their impotance?

Answer 1

Fraunhofer lines named after the German physicist Joseph von Fraunhofer are a set of spectral lines. These lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.
They have their well defined wavelengths because of which they are often used to characterize the refractive index and dispersion properties of optical materials.