1. Lenses and Optical Devices
Chapter 12
Lenses and Optical Devices

2. Key Ideas
KEY QUESTION: How do lenses produce images, and how do lenses benefit humans?
4 Lessons
Lenses and the Formation of Images
 Images in Lenses
The Lens Equations
The Human Eye

3. Learning Goal To understand the main characteristics of Lenses
To understand how images form in converging or diverging lenses

4. Basic Lens Shapes Lenses consist of two basic shapes Converging lens
where parallel light rays converge through a single point after refraction through the lens
Is thickest in the middle and thinnest at the edge
Diverging lens
Where parallel light rays diverge after refraction from the lens
Is thinnest in the middle and thickest at the edge.

5. Basic Lens Shapes

6. Light Rays & Lenses
In a lens, light is refracted first at the air to glass surface
Light then travels through the glass of the lens and is refracted again at the glass to air surface on the other side
This means that there are always two refractions in a lens
Yet, we only look at the direction of the incident ray entering the lens and the ray leaving the lens

7. Light Rays & Lenses
Ray diagrams can be simplified by drawing a dashed vertical line through the centre of the lens and showing refraction occurring at this line
The central line is a reference point and shows light being refracted only once

8. Converging Lenses
The centre of the lens is called the optical centre (O)
The line through the optical centre that is perpendicular to the central dashed line of the lens is the principal axis
Light rays parallel to the principal axis converge through a single point on the principal axis called the principal focus (F)

9. Converging Lenses
Light can strike the lens from either side, and both sides of the lens can focus parallel light rays
To tell them apart, the focus that is on the same side of the lens relative to the incident rays is usually labelled as the secondary principal focus (F ʹ)

10. Converging Lenses

11. Diverging Lenses
Light rays parallel to the principal axis of a diverging lens do not converge
Instead, the refracted rays spread apart
If you project these diverging rays backwards, it looks as if they come from a virtual focus
This point is now the principal focus (F)

12. Diverging Lenses
The secondary principal focus (F ʹ) is now on the other side of the lens, where the rays actually diverge
Note that F and F ʹ are equally far apart from the optical centre of both a converging lens and a diverging lens

13. Diverging Lenses