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Chapter 13: Lenses and
Optical Devices
13.1 Lenses and the
Formation of Images
Basic Lens Shapes
• Converging Lens
– Parallel light rays
converge through a
single point after they
travel through the lens.
– Lens is thickest in the
middle.
• Diverging Lens
– Parallel light rays
diverge after they
travel through the lens.
– Lens is thickest at the
edge.
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Converge
Diverge
Lenses use Refraction
• Light is refracted twice as it passes through a lens:
once from air to glass, and once from glass to air.
• Ray diagrams are simplified by drawing a line through
the center of the lens and showing refraction occurring
at this line.
Converging Lenses
• The Optical Center (O) is point
at the exact centre of the lens.
• The Principal Focus (F) is the
point at the principal axis of a
lens where light rays parallel to
the principal axis converge after
refraction.
• The Secondary Principal
Focus (F’) is on the same side
of the lens relative to the incident
rays. F and F’ are at an equal
distance to the optical centre.
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Converging Lens
•A converging lens brings parallel light rays together through a principle focus (F)
after refraction.
•The principal focus of a converging lens is on the opposite side of the lens as the
incident rays.
Terminology of Diverging Lenses
• Light rays of a diverging
lens do not converge. If
you project these
diverging rays
backwards, it looks as if
they come from a virtual
focus. This is the
principle focus (F).
• The secondary focus (F’)
is on the other side of the
lens where the rays
actually diverge.
Diverging Lens
Homework / Classwork
P. 553 # 2, 3, 5,
6