Textbook Chp 12.4 Pg 237 to 247
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Converging and Diverging Lenses
Focal Length of a Converging Lens
Ray Diagrams for Converging Lens
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In this classroom there are no less than 4 kinds
of lenses. What are they?
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Source: blundelloptometry.com
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Source: techradar.com
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Diverging Light Rays
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Converging Light Rays
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Meet at a point
Parallel Light Rays
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“Beam” of light
Light from distant objects are
approximately parallel
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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
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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
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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
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Definition: Focal length f is the
distance between the optical centre
and the focal point.
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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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The Human Eye
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Cooking only using sunlight
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http://youtu.be/gvozcv8pS3c
http://www.youtube.com/watch?v=wprlzCDfLm8
Lasik Surgery Explained
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http://www.youtube.com/watch?v=n54johuXzF4
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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:
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Rule 1: Any ray passing through the optical
centre passes through the lens without bending
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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
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Rule 3: An incident ray passing through the
focal point F is refracted parallel to the
principle axis
F
F
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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
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You need to be familiar with all 6 case studies
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Each of them follow the 3 rules
For each case study, you need to be able to
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u = far away, u > 2f, u = 2f, 2f < u < f, u = f, u < f
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.
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Real or Virtual?
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Upright of Inverted?
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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
For a converging lens, real images are always
inverted. Virtual images are upright.
Magnified or Diminished?
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Bigger or smaller than the real object?
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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?
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[Not in syllabus]
The word “lens” comes from the word “lentil”
Lentils are a kind of bean which has a doubleconvex shape – just like what converging lenses
look like
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[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
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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)
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If you have not understood the concepts
thought in Light Part 3, put a gold paperclip on
your personal card.