Opacity occurs because of the reflection of light waves off the surface of an object. When light waves are reflected, an object appears opaque.

What stuff are opaque?

An opaque object is something that allows no light through it. Concrete, wood, and metal are some examples of opaque materials. Some materials can be opaque to light, but not to other types of electromagnetic waves.

Why do opaque objects absorb light?

If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then reemit the energy as a reflected light wave.