Lenses are basically panes of glass whose entrance and exit surfaces are
not parallel. This causes the direction of the light beam that enters them
to change. Lenses can be divided into two basic groups: those that cause
light rays to diverge, and those that cause them to converge; if they are
thicker in the middle, they are converging, if they are thicker on the
sides, they are diverging. All lenses have two focal points; one on each
side. In a converging lens, light coming in parallel to the optic axis is
focused at a point on the other side called the principal focal point. The
distance from this focal point to the center of the lens is called the
focal length. To find the focal point on the opposite, just shine the light
in the other direction. On a diverging lens, the principal focal point is
found on the same side from which the light was shown; that is, the point
where the rays on the other side seem to come from. In a lens, the focal
point that is not the principal one is referred to as the "other"
Finding the image of objects through a lens is very similar to that of the finding the image involving mirrors. Once again, three rays are drawn, and where they meet (or their extensions meet), the image is formed. First, draw a ray from your point on the object through the center of the lens, and just continue it. Secondly, draw a ray coming from the point going parallel to the optic axis toward the lens; it should go through (or from, in some instances) the principal focal point. Finally, draw a ray through (sometimes, towards) the other focal point that is parallel to the optic axis.
On a converging lens, if the object is outside of the focal point, the image that appears will be real. If it is inside one of the focal points, the image that appears will be virtual, and acutally behind the object. On a diverging lens, images created are always virtual.