SNC2D1: LENSES
Basic Lens Shape
Lenses consist of two basic shapes. The first is a converging
(convex) lens, so named because parallel light rays converge to a
single point after refraction through the lens. See fig 1. The
converging lens is thickest in the middle and thinnest at the edge.
Fig. 1
Fig. 2
The second kind of lens is a diverging (concave) lens. In a diverging
lens, parallel light rays diverge after refraction from the lens. See fig
2. A diverging lens is thinnest in the middle and thickest at the
edge.
Terminology of Converging Lenses
 The optical centre, O, is the centre of the lens.
 The principal axis, PA, is a line drawn through O that is
perpendicular to the surface of the lens.
 The principal focus, F, is the point where the parallel rays
converge after passing through the lens.
 Light can strike the lens from either side, and both sides of
the lens can focus parallel light rays. Therefore there are two
focal points. To tell them apart, the focal point located on
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the same side of the object where the incident rays occurs is
usually called the secondary principal focus, F’.
 The focal length, f is the distance from the optical centre to
the focal point.
Terminology of Diverging Lenses
Remember that light rays parallel to the principal axis of a diverging
lens do not converge. Instead, the refracted rays spread apart. If
you were to extend these light rays backwards it would appear as if
they originated from the principal focus.
 The optical centre, O, is the centre of the lens.
 The principal axis, PA, is a line drawn through O that is
perpendicular to the surface of the lens.
 The principal focus, F, is the point where the refracted rays
cross if they were to be extended. The secondary principal
focus, F’ is now located on the other side of the lens where
the rays actually diverge.
Note: F and F’ are equally far apart from the optical centre of both
converging and diverging lens.
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How To Locate The Image In A Converging Lens
The following rules can be used for locating images in a converging
image.
1. A ray parallel to the
principal axis is refracted
through the principal focus
(f).
2. A ray through the secondary
principal focus (f’) is
refracted
parallel are
to the
Note that
all images
drawn where refracted rays meet or cross.
principal axis.
3. A ray through the optical
Outlined on the next page is how images are formed
in a converging
lens.
How To Locate The Image In A Diverging Lens
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centre (O) continues
straight through without
being refracted.
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The imaging rules for a diverging lens are similar to those for a
converging lens. The only difference is that light rays do not
actually come from the principal axis – F; they only appear to.
1. A ray parallel to the principal
axis is refracted as if it had
come through the principal
focus (f)
2. A Ray that appears to pass
through the secondary
principal focus (f’) is
refracted parallel to the
principal axis.
3. A ray through the optical
centre (O) continues
straight through on its path
Images In A
Diverging Lens
A diverging lens always produces the same image characteristics no
matter where the object is. The image is always smaller, upright,
virtual and on the same side of the lens as the object.
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SNC 2D Activity: Locating Images
1. Add light rays to the diagrams to locate the image for each
object. Describe the SALT for each diagram.
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