Light / optics – part 3 Lenses

1. Light / optics – part 3Lenses Textbook Chp 12.4 Pg 237 to 247

2. Sub-topics • Converging and Diverging Lenses • Focal Length of a Converging Lens • Ray Diagrams for Converging Lens

3. Before We Begin • In this classroom there are no less than 4 kinds of lenses. What are they? Source: blundelloptometry.com Source: baliweddingsolutions.com Source: porhomme.com Source: techradar.com

4. Previously… • Diverging Light Rays • Converging Light Rays • Meet at a point • Parallel Light Rays • “Beam” of light • Light from distant objects are approximately parallel

5. Diverging Lens • When parallel rays passes through a diverging lens (concave lens), the rays become diverging rays (because of refraction) • Glasses for short-sightedness are an example of diverging lenses • Note that you cannot use your glasses to focus sunlight like a magnifying glass

6. Converging Lens • When parallel rays passes through a diverging lens (convex lens), the rays become converging rays (refraction again) Focal Plane Focal Point Optical Centre Focal Length, f

7. Converging Lens • The focal point is fixed for any one lens • If a converging lens is symmetrical, then the focal point on either side of the lens is the same distance away • The line joining the optical centre and the focal points is called the principal axis. Focal Point Focal Point Principal Axis

8. Focal Length of a converging lens • Definition: Focal length f is the distance between the optical centre and the focal point.

9. Terminology • The terms “concave” and “convex” are no longer in your syllabus. • They refer to the shape of the lens (or any object) • Focal Point may also be called “focus point” or “principal focus” by other teachers/textbooks • Don’t need to remember definition, but need to be familiar with terms: optical centre, focal point, principal axis, focal plane

10. Half Time! • The Human Eye • http://youtu.be/gvozcv8pS3c • Cooking only using sunlight • http://www.youtube.com/watch?v=wprlzCDfLm8 • Lasik Surgery Explained • http://www.youtube.com/watch?v=n54johuXzF4

11. Ray Diagrams for Converging Rays • There are 3 rules (pg 240) for drawing ray diagrams and 6 case studies (pg 242) • Note: a symbol often used for converging lens is the vertical double-headed arrow:

12. Ray Diagrams for Converging Rays • Rule 1: Any ray passing through the optical centre passes through the lens without bending

13. Ray Diagrams for Converging Rays • Rule 2: An incident ray parallel to the principle axis is refracted to pass through the focal point F • Application – using magnifying glass to burn things F Principal Axis

14. Ray Diagrams for Converging Rays • Rule 3: An incident ray passing through the focal point F is refracted parallel to the principle axis F F

15. Recap 3 Rules • Rule 1: Any ray passing through the optical centre passes through the lens without bending • Rule 2: An incident ray parallel to the principle axis is refracted to pass through the focal point F • Rule 3: An incident ray passing through the focal point F is refracted parallel to the principle axis

16. 6 case studies • You need to be familiar with all 6 case studies • u = far away, u > 2f, u = 2f, 2f < u < f, u = f, u < f • Each of them follow the 3 rules • For each case study, you need to be able to • Describe the image properties • Give real life examples of each case study • Recommend: do NOT memorize the 6 case studies, but instead learn how to draw them from scratch.

17. Image Properties • Real or Virtual? • Real images are formed by real light rays, and can be captured on a screen • Virtual images cannot be captured on a screen • Virtual images are on the same side of the lens as the object • Upright of Inverted? • For a converging lens, real images are always inverted. Virtual images are upright. • Magnified or Diminished? • Bigger or smaller than the real object?

18. 6 case studies • Draw on graph paper! • Refer to pg 242 if necessary • Test yourself – are you able to draw all 6 case studies & remember their image properties and applications without referring to notes or textbook?

19. Did You Know… • [Not in syllabus] • The word “lens” comes from the word “lentil” • Lentils are a kind of bean which has a double-convex shape – just like what converging lenses look like

20. For Further Research • [Not in syllabus] • 1) What is a Fresnel lens? • 2) How can someone be long-sighted and short-sighted at the same time? (i.e. how come some old people wear bifocal glasses?) Bifocal glasses were invented by Benjamin Franklin. Source: oobject.com

21. Recap • Diverging lens and Converging lens • Optical Centre, Focal Point (F), focal plane and focal length (f) • Focal length f is the distance between the optical centre and the focal point. • 3 rules of drawing ray diagram • 6 case studies (each case study must be able to draw, describe image properties, and provide real-life use)

22. Personal Card Feedback • If you have not understood the concepts thought in Light Part 3, put a gold paperclip on your personal card.