Luminous bodies are of two kinds; those which shine by their own light, and those which shine by reflected light.
Transparent bodies are such as permit rays of light to pass through them.
Translucent bodies permit light to pass faintly, but without representing the figure of objects seen through them.
Opaque bodies permit no light to pass through them, but reflect light.
A ray is a line of light.
A beam is a collection of parallel rays.
A pencil is a collection of converging, or diverging rays.
A medium is any space through which light passes.
Incident rays are those which fall upon the surface of a body.
Reflected rays are those which are thrown off from a body.