prevent LIGHT 

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

we can see through clear glass

Block out

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.

Shadows are created when light cannot penetrate an object. The risers (vertical part of the step) appear dark in the iamge opposite - there is little light reflecting from them. 

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