Relief is a rough measure of the difference in refractive index of the glass and that of the medium.
When the difference in refractive indices is negligible, the particle edges and surface features are difficult to see because the degree of relief is low. In this situation, there is no contrast because the light doesn't change speed or angle when it travels from medium to particle. As the difference in index of refraction is increased, the boundary between particle and medium becomes darker and more pronounced due to an increase in reflected and refracted light rays. It is important to note that the intensity of the incoming light is not affected by the difference in refractive index, but rather light is refracted so that less light reaches the objective. The photos below illustrate the three levels of relief.