Light is prevented from moving through a surface if it is opaque.

we can see through clear glass

When the view is blocked light is prevented from going through

Meet Preemo Prevent!

Curtains and blinds will prevent light from entering a space. The top of the window can be encased with a box pelmet to stop the light from penetrating above the curtain.

Sometimes there is some light reflecting from the surface, so it appears greyer than where the light is reflecting more.

Shadows are created when light cannot penetrate an object

The risers (vertical part of the step) appear dark in the image opposite - there is little light reflecting from them.