1. Lenses
A lens is a transparent material (with at least one curved side) that causes light refracts in a predictable and useful way.
Each ray is refracted by a different amount at each surface.
Rays striking the lens near the edge are bent the most while rays near the centre experience the least amount of bending.
The image produced by a lens can be real or virtual, inverted or upright, larger or smaller.

2. Converging and Diverging Lenses
Converging (convex ) Lenses
Diverging (concave ) Lenses

3. Lenses have a focal point akin to curved mirrors.
Because there are two sides to a lens, there are two focal points: F & F’ (more later!)
Converging Lens F is behind the lens
Diverging Lens F is in front of the lens
Incident rays parallel to the principal axis intersect at the focal point.

4. Ray Diagrams: Converging Lens
Object
P.A.
Image
Incident ray parallel to principal axis will exit through F.
Incident ray through F’ will exit parallel to the principal axis
Incident ray through centre of lens (intersection of axis of symmetry and P.A.) do not change direction.

5. Ray Diagrams: Converging Lens
Object
P.A.
Image
S – Smaller than object A - Inverted L – Closer to lens than object T - Real

6. Ray Diagrams: Diverging Lens
Object
P.A.
Image
Incident ray parallel to principal axis is refracted so that it appears to pass through F.
Incident ray appears to pass through F’ is refracted parallel to the principal axis.
Incident ray through centre of lens (intersection of axis of symmetry and P.A.) do not change direction.

7. Ray Diagrams: Diverging Lens
Object
P.A.
Image
S – smaller than object A - upright L – closer to lens than object T - virtual