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15.1 Lenses
What is a lens?
A contact lens
Can you list out
other applications
of lenses?
Lenses are used in
telescopes
microscopes
cameras
15.1 Lenses
Convex and concave lenses
convex lenses
thicker at the centre
than at the edge
concave lenses
thinner at the centre
than at the edge
15.1 Lenses
What happens when
parallel light rays pass
through a convex lens?
The rays converge to a single point.
convex lens = converging lens
15.1 Lenses
What happens when
parallel light rays pass
through a concave lens?
The rays diverge and appear to
come from a single point.
concave lens = diverging lens
15.1 Lenses
Terms used to describe convex lenses
optical centre
C
optical centre – the centre of the lens
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principal
axis
C
principal axis – a straight line perpendicular
to the lens and passing
through the optical centre
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principal focus
F’
C
F
 Rays passing through convex lens
parallel to the principal axis converge
to the principal focus.
 A convex lens has 2 principal foci.
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F’
C
F
focal length f
focal length – the distance between the
focus and the optical centre
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focal plane
principal
axis
C
F
focal plane – a plane through the principal
focus and is perpendicular to
the principal axis.
Simulation: Parallel rays through convex lens
15.1 Lenses
Terms used to describe concave lenses
principal focus
principal
axis
F’
focal length f
optical centre
C
F
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C
principal axis
F’
focal plane
Simulation: Parallel rays through concave lens
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How lenses work
Why does a convex
lens converge light rays?
Why does a concave
lens diverge light rays?
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Think of a convex lens as a number of prisms.
The rays near the edge
are refracted more.
F
When light rays leave the
prisms, they are refracted
away from the normal.
When light rays enter the
prisms, they are refracted
towards the normal.
15.1 Lenses
Think of a concave lens as a number of prisms.
The rays near
the edge are
refracted more.
F’
When light rays
enter the prisms,
they are refracted
towards the normal.
When light rays leave the
prisms, they are refracted
away from the normal.
15.1 Lenses
Does the ray passing
through the optical centre of
a lens change direction?
No. It is laterally displaced.
larger lateral
displacement
smaller lateral
displacement
15.1 Lenses
That’s the end of Section 15.1
Check Point
Key Ideas
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