Why are there different degrees of relief?
The greater the difference between the refractive indices, the greater the refraction of the waves. This means that their divergence is greater, thus leading to a wider dark area surrounding the mineral.
This dark line has the optical effect of making the image of the mineral stand out from the medium around it.
This is called relief and is defined as the intensity with which a particular mineral stands out from the material which surrounds it in a microscopic image.
Relief is a consequence of the difference between the refractive indices of the two mediums in contact. When there is only slight relief, the difference is small, whilst when the relief is sharp, there exists a significant difference between the indices of the mineral and the medium.