How is a shadow related to absorption?

Answer 1

A shadow is what we call the absence of light due to some object coming in between the light source and surface on which the "shadow" falls.

The 'object' which was in the way of the light could be, for example, solid rock or colored glass.

Both block the direct path of the light, but at different rates and in different ways.

The rock, would absorb a great proportion of the light due to its high density.

The glass however, is less dense, therefore light passes through it more freely, with less of it being absorbed. Therefore more of it continuing to finally reach the surface.

So to a certain degree, shadows are directly related to the physical makeup of the shadow casting object. The more the object absorbs, the darker the shadow will be and vice versa.

There are phenomena I've chosen to leave out of my answer for simplicity. But you could start looking into volume properties and terms such as scattering, emission, extinction, optical density in addition to absorption.