1. Lenses
Topic 13.4

2. Outcomes
You will use ray diagrams to describe an image formed by thin lenses and curved mirrors.
You will describe, quantitatively, simple optical systems, consisting of only one component, for both lenses and curved mirrors.

3. Lens Terminology
All the terms from the mirror section also apply to lenses, however instead of a centre of curvature there is an optical centre.
Optical Centre: The center of the lens. The principal axis passes through this point

4. Comparing Lenses to Mirrors
Light can travel either direction through the lens, there is a focal point on both sides of the lens.
Convex lenses have a real focal point (aka converging lenses)
Concave lenses have a virtual focal point (aka diverging lenses)
Focal length is dependent on the type of material the lens is made of – the smaller the index of refraction the greater the focal length

5. Converging lenses have a positive focal length.
VIRTUAL FOCAL POINT!
REAL FOCAL POINT!
Converging lenses have a positive focal length.
Diverging lenses have a negative focal length.

6. Drawing Ray Diagrams Steps:
1. Draw the lens showing the principle axis and both principal focal points
2. Place a vertical arrow on the principal axis to illustrate the position and size of the object.
3. Draw two rays from the tip of the object arrow to the lens.
The first ray is drawn from the tip of the object parallel to the principal axis and will refract through (or appear to diverge from) the principal focal point.
The second ray is drawn from the tip of the object arrow straight through the optical center of the lens

7. Calculating Lens Characteristics
The same equations and principles are as those for mirrors

8. IMPORTANT RULES *Note:
Converging lenses have a positive focal length.
Diverging lenses have a negative focal length.
*Note:
All real images are inverted and all virtual images are erect
Do and ho are always +

9. Sample Problem
A 25 cm tall object is placed 40 cm from a convex lens of focal length 50 cm. Calculate di, magnification, and hi.

10. Homework
Drawing Lens Diagrams Handout
Calculation Qs: p. 116 #2-12