LIGHT / OPTICS – PART 3

LENSESTextbook Chp 12.4 Pg 237 to 247

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

Before We Begin 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

Previously… Diverging Light Rays

Converging Light Rays Meet at a point

Parallel Light Rays “Beam” of light Light from distant objects

are approximately parallel

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

Converging Lens 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

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

Focal Length of a converging lens

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

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

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

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:

Ray Diagrams for Converging Rays

Rule 1: Any ray passing through the optical centre passes through the lens without bending

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

Principal AxisF

Ray Diagrams for Converging Rays

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

FF

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

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.

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?

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?

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

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

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)

Personal Card Feedback If you have not understood the concepts

thought in Light Part 3, put a gold paperclip on your personal card.