1. Introduction to Lenses

2. Definition of a Lens
A lens is transparent piece of plastic or glass with at least one refractive surface.

3. How do lens work
A lens works by refracting (bending) light rays as they pass through it so they change direction
Lenses work by bending light at a glass-air boundary.  Although refraction occurs at the boundary, we will treat all lenses as bending the rays at the lens axis.

4. Converging Lens
The lens in the eye is a convex or converging lens.  This means that the lens makes rays of light come together, or converge.

5. Converging Lens
The rays parallel to the principal axis are converged onto the principal focus.  
The focal length is the distance between the lens axis and the principal focus.

6. Diverging Lens
The concave lens cause rays traveling parallel to the principal axis to diverge, which is why it is called a diverging lens.
The principal focus is virtual because the rays do not pass through it, but diverge as if they had come from it.