Textbook Chp 12.4 Pg 237 to 247

 Converging and Diverging Lenses  Focal Length of a Converging Lens  Ray Diagrams for Converging Lens

 In this classroom there are no less than 4 kinds of lenses. What are they? Source: baliweddingsolutions.com Source: porhomme.com Source: blundelloptometry.com Source: techradar.com

 Diverging Light Rays  Converging Light Rays  Meet at a point  Parallel Light Rays  “Beam” of light  Light from distant objects are approximately parallel

 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

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

 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 Principal Axis

 Definition: Focal length f is the distance between the optical centre and the focal point.

 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

 The Human Eye   Cooking only using sunlight   Lasik Surgery Explained 

 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:

 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  Application – using magnifying glass to burn things Principal Axis F

 Rule 3: An incident ray passing through the focal point F is refracted parallel to the principle axis F F

 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

 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.

 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?

 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?

 [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

 [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

 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)

 If you have not understood the concepts thought in Light Part 3, put a gold paperclip on your personal card.