1. Convex and Concave Lenses
Types of Lenses
A lens is a piece of transparent material, such as glass or plastic, that is used to focus light and form an image.
Each of a lens’s two faces might be either curved or flat.

2. Convex and Concave Lenses
Types of Lenses
The lens shown in the figure is called a convex lens because it is thicker at the center than at the edges.
A convex lens often is called a converging lens because when surrounded by material with a lower index of refraction, it refracts parallel light rays so that the rays meet at a point.

3. Converging Lenses Focal point Principal axis Optical center
Focal plane
Focal length

4. Ray Diagrams F F

5. Rays to be Drawn
Principal - Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.
Focal - Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.
Central - An incident ray that passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

6. Ray Diagram

7. Ray diagrams for a double convex lens
Object is at infinity F

8. Object beyond 2F F 2F 2F 2F

9. Object at 2F 2F 2F

10. Object between F and 2F F 2F 2F F

11. Object at F F F

12. Object between F and the lens

13. Images Formed by Lens Object distance Type of image Uses d = 
Inverted, smaller, real
Telescope
d > 2f
Camera, eye
d = 2f
Inverted, same size, real
Photocopier
f < d < 2f
Inverted, magnified, real
Projector
d = f
upright, magnified, real
Spotlight
d < f
upright, magnified, virtual
Magnifying glass