- Transparent: material that allows all light to pass through
- Translucent: material that allows some light to pass through
- Opaque: material that allows no light to pass through
Refraction is the bending of light rays when passing through a surface between one transparent material and another. Reflection is the throwing back by a body or surface of light (or heat or sound) without absorbing it. It is the change in direction of a wavefront at an interface between two different materials (media) so that the wavefront returns into the material (medium) that it originated from.
Reflection vs. Refraction
Concave means "hollowed out or rounded inward" and can be easily remembered by keeping in mind that these surfaces "cave" in. The opposite is convex, meaning "curved or rounded outward."