When the light source is turned on, the scientist observes that no light is detected on the far side of the substance. What is the best interpretation for this observation?

The light is either absorbed or reflected

Explanation:

When a ray of light is incident on any surface, three possible things happens, either it gets reflected, absorbed or get transmitted through the surface. This light is detected on the far side of the surface of the substance depending on the transparency of the surface.

Since the condition here is given that there occurs no light on the far side or the other side, so the light must be either reflected by the surface or the light must be absorbed entirely by the surface.