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.

Parallel rays are such as proceed equally distant from each other
through their whole course.

Converging rays are such as approach and tend to unite at any one point,
as at b. fig. 3.

Diverging rays are those which continue to recede from each other,
as at e. Fig. 3.

A Focus is that point at which converging rays meet.

MOTION OF LIGHT--Rays of light are thrown off from luminous bodies
in every direction, but always in straight lines, which cross each
other at every point; but the particles of which each ray consists
are so minute that the rays do not appear to be impeded by each other.
A ray of light passing through an aperture into a dark room,
proceeds in a straight line; a fact of which any one may be convinced
by going into a darkened room and admiting light only through
a small aperture.

Light also moves with great velocity, but becomes fainter as it recedes
from the source from which it eminates; in other words, diverging rays
of light diminish in intensity as the square of the distance increases.