SNC2D
12.2 – IMAGES FORMED BY LENSES
 The ______________________________________ is an imaginary line drawn through the
optical centre perpendicular to both surfaces.
 The ______________________________________ is an imaginary vertical line drawn
through the optical centre of a lens.
 Both types of lenses have ___________________________________________.
 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.
 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.
 The light rays undergo two refractions:
 1st on _______________________ the lens
 2nd on _______________________ the lens.
 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 LENS
 A diverging lens is sometimes called a _________________________________ 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 ___________________ meet on the other
side of the lens.
 You may remember “diverging” as “dividing”.
 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.
SNC2D
12.2 – IMAGES FORMED BY LENSES
Locating Images in Diverging Lens (Follow along with Table 12.2 on page 497 of the textbook)
Any ___________________ 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 ______________________________________________ are needed to find the image.
Drawing a Diverging Lens Ray Diagram
 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 ___________________________________ 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”.
 Centre of the lens is called the _____________________________________________, O.
 Refracted rays meet at a point called the _____________________________________, 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.
 Both F and F’ are equal distance from the optical centre.
 The line through the optical centre and the two foci is called the ______________________.
 The distance between F to O is the focal length, f, of the lens.
SNC2D
12.2 – IMAGES FORMED BY 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.
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
LOCATION
TYPE