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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?
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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
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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 Axis
F
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)