Section 12.2
Pages 494 - 498
Lens Terminology
 The principal axis is an
imaginary line drawn
through the optical
centre perpendicular to
both surfaces.
 The axis of symmetry
is an imaginary vertical
line drawn through the
optical centre of a lens.
Lens Terminology
 Both types of lenses
have two principal
focuses.
 The focal point where
the light either comes to
a focus or appears to
diverge from a focus is
given the symbol F,
while that on the
opposite side of the lens
is represented by Fʹ.
Lens Terminology
 The focal length, f, is
the distance from the
axis of symmetry to the
principal focus
measured along the
principal axis.
 Both types of thin lenses
have two equal focal
lengths.
Drawing a Ray Diagram for a Lens
 The light rays will bend, or refract, away from the lens
surface and toward the normal.
 When the light passes out of the lens at an angle, the
light rays refract again, this time bending away from the
normal.
 The light rays undergo two refractions:
 1st on entering the lens
 2nd on leaving the lens.
When Drawing Ray Diagrams for a
Lens ... Keep in mind ...
 A thin lens is a lens that has a thickness that is slight
compared to its focal length. An example of a thin lens
is an eyeglass lens.
 You can simplify drawing a ray diagram of a thin lens
without affecting its accuracy by assuming that all the
refraction takes place at the axis of symmetry.
DIVERGING LENSES
 A diverging lens is sometimes called a
concave lens because it is thinner in
the centre than at the edges.
 As parallel light rays pass through a
concave lens, they are refracted away
from the principal axis.
 The light rays spread apart (diverge)
and they will never meet on the other
side of the lens.
 You may remember “diverging” as
“dividing”.
PROPERTIES OF DIVERGING LENSES
 Refracted rays appear to spread from a virtual focus called the
principal focus, F.
 It is on the same side as the incident rays.
 The secondary principal focus, F’, is located on the other side of
the lens.
 Focal length (f), optical centre (O), and principal axis are the same
as in a converging lens.
O
Principal axis
F’
Focal length (f)
Principal focus
Locating Images in Diverging Lens
 The image rules for diverging lenses are similar to
those for a converging lens. The only difference is
that light rays do not actually come from the principal
focus (F); they only appear to.
 Follow along with Table 12.2 on page 497 of the
textbook.
Locating Images in Diverging
Lenses
Any two of the following rays may be used to locate the
image:
1. Draw a ray parallel to the principal axis that is
refracted through the principal focus (F).
2. Draw a ray that passes through the secondary
principal focus (F') and refracts parallel to the
principal axis.
3. A ray that passes through the optical center goes
straight through, without bending.
Only two of these lines are needed to find the image.
Drawing a Diverging Lens Ray
Diagram
2F
S: Smaller
F’’
F
A: Upright
2F’
L: Between F and
axis of symmetry
T: Virtual
Ray Diagrams - Diverging Lenses
 Diverging lenses always produce the same images.
 Smaller, upright, virtual, and on the same side of the
lens as the object.
 As the object moves farther from the lens, the image
becomes smaller.
CONVERGING LENSES
 A converging lens is also called a
convex lens because it is thicker at
the centre than at the edges.
 All incident parallel light rays
converge at a single point after
refraction.
 You may remember “converging” as
“concentrating”.
Converging lenses are often
used as magnifying glasses.
PROPERTIES OF CONVERGING LENSES
 Centre of the lens is called the optical centre, O.
 Refracted rays meet at a point called the principal focus, F.
 It is located on the opposite side of the incident rays.
 Light can strike the lens from either side, and both sides can focus
parallel rays. Thus, there can be a secondary principal focus, F’,
on the same side as the incident rays.
Principal axis
F’
O
Focal length (f)
Principal focus
PROPERTIES OF CONVERGING LENSES
 Both F and F’ are equal distance from the optical centre.
 The line through the optical centre and the two foci is
called the principal axis.
 The distance between F to O is the focal length, f, of the
lens.
Principal axis
F’
O
Focal length (f)
Principal focus
PROPERTIES OF CONVERGING LENSES
 Light can strike the lens from either side and both sides
can focus parallel rays.
 Thus, there can be a secondary principal focus, F’, on
the same side as the incident rays.
 Both F and F’ are equal distance from the optical centre.
Principal axis
F’
O
Focal length (f)
Principal focus
Drawing a Converging Lens Ray
Diagram
1. Any ray parallel to the principal axis is refracted
through the principal focus (F).
2. A ray that passes through the secondary principal
focus (F') is refracted parallel to the principal axis.
3. A ray that passes through the optical center goes
straight through, without being refracted (bending).
As with converging mirrors, only two rays are required to
locate an image. The third one acts as a check.
SUMMARY: IMAGING PROPERTIES
OF A CONVERGING LENS
OBJECT
LOCATION
Beyond 2F’
At 2F’
Between 2F’
and F’
At F’
Between F’
and mirror
IMAGE CHARACTERISTICS
SIZE
ATTITUDE
(smaller, larger, or same)
(inverted or upright)
LOCATION
TYPE
(Real or virtual)
JIGSAW
There are 5 scenarios to investigate with CONVERGING
LENS.
You will be placed in groups and assigned a scenario to
work on, during which time you will become the ‘expert’
on that scenario.
Afterwards, everyone will JIGSAW into another group
where you will all have a turn to ‘teach’ your new group
members about your converging lens scenario.
Please reference your textbook (p.495) during this activity.
JIGSAW GROUPS
GROUP 1
Adoma
Annamaria
Helder
Vanessa
David
Stephen
GROUP 2
Julia
Anthony
Stephanie
Jonathan
Abdullah
Raffaela
GROUP 3
Akin
Victor
Michael
Alissia
Higor
Rachel
GROUP 4
Haya
Andre
Ivo
Josie
Matthew
GROUP 5
Angelica
Alex
Cristian
Gabriel
Sevak
Victoria
SUMMARY: IMAGING PROPERTIES
OF A CONVERGING LENS
OBJECT
LOCATION
Beyond 2F’
At 2F’
Between 2F’
and F’
At F’
Between F’
and mirror
IMAGE CHARACTERISTICS
SIZE
ATTITUDE
(smaller, larger, or same)
(inverted or upright)
LOCATION
TYPE
(Real or virtual)
SUMMARY: IMAGING PROPERTIES
OF A CONVERGING LENS
OBJECT
LOCATION
IMAGE CHARACTERISTICS
SIZE
ATTITUDE
(smaller, larger, or same)
(inverted or upright)
LOCATION
TYPE
(Real or virtual)
Beyond 2F’
Smaller
Inverted
Between F
and 2F
Real
At 2F’
Same size
Inverted
At 2F
Real
Between 2F’
and F’
Larger
Inverted
Beyond 2F
Real
At F’
NO CLEAR IMAGE FORMED
(Emergent rays are parallel)
Between F’
and mirror
Larger
Upright
Virtual
Solutions for JIGSAW Diagrams
Object between 2F’ and F’
2F’
S: Larger
F’
A: Inverted
F
L: Beyond 2F
2F
T: Real
Object beyond 2F’ (An object more than two times the
distance of the focal length from the lens)
2F’
S: Smaller
F’
A: Inverted
F
2F
L: Between F
and 2F
T: Real
Object at 2F’
2F’
F’
S: Same size A: Inverted
F
L: At 2F
2F
T: Real
Object at F’
2F’
F’
 NO IMAGE FORMED
F
2F
Object in front of F’
2F’
S: Larger
F’
A: Upright
F
L: Behind F’
2F
T: Virtual
Solutions for
Converging Lens