The double convex lens/double convex mirror is symmetrical across both its horizontal and vertical axis. Each of the lens's two faces can be thought of as originally being part of a sphere. The fact that a double convex lens/mirror is thicker across its middle is an indicator that it will converge rays of light that travel parallel to its principal axis. A double convex lens/mirror is a converging lens. A double concave lens/mirror is also symmetrical across both its horizontal and vertical axis. The two faces of a double concave lens can be thought of as originally being part of a sphere. The fact that a double concave lens is thinner across its middle is an indicator that it will diverge rays of light that travel parallel to its principal axis. A ddouble convex lenses/mirror is a diverging lens. These two types of lenses - a double convex and a double concave lens/mirror will be the only types of lenses that will be discussed in this unit of The Physics Classroom Tutorial.