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Focussing Light onto the retina
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WALT
How light rays are focussed onto the retina
How the cornea and lens refract light rays
Accommodation of the eye
Controlling light
levels
• Your eye are very
sensitive and can
be damaged by
harsh light.
• Your iris controls
light allowed into
the eye by
changing the size
of the pupil
The colored part of the eye is called
the iris.
It controls light levels inside the eye
similar to the aperture on a camera.
Radial and circular muscles in the
iris control pupil size in response to
light
The round opening in the center of
the iris is called the pupil.
Control of Pupil Size
Focusing
on objects
• The lens and
cornea focus
the light on
the retina
Focusing
• The lens job
is to make the
rays hit the
same point
The red rays will
be out of
focus
Bending light
• Light is refracted and
bent to focus it as it
passes through the
lens
• Lens thickness can
be changed so the
amount of bending is
changed
Near vision
• The lens
needs to
bend the
light more
to focus it
• Fat lens
needed
Distance vision
• Rays enter the
eye closer
together
• Need less
bending
• Thinner lens
needed
Changing lens thickness
Ciliary muscles are attached to the lens, when relaxed
they pull on the suspensory ligaments causing the
lens to be thin
When contracted the ciliary muscle cause the
suspensory ligaments to slacken and so the lens is
fatter
Accommodation
• When we open our eyes in the morning they are
not focussed on near by objects
• At rest the ciliary muscles relax , puling the lens
flat
• In this state we can focus on distant objects
• To focus on something near by the ciliary
muscles contract reducing the tension on the
suspensory ligaments so the lens gets fatter
• This is called accommodation – changing the
shape of the lens to focus on near or distant
objects
When the eye looks at an object that is far away the ciliary
muscle relaxes which pulls on the suspensory ligaments.
These pull on the lens and make it flatter (less convex). This
brings the rays of light from the object into crisp focus on
the retina.
Distant Objects
With an object closer to the eye, the lens needs to be
more convex (fatter). To do this, the ciliary muscles
contract making the suspensory ligaments slack and
to allow the rubbery lens to return to its naturally
rounder shape.
Near Objects
Myopia
(short sightedness)
the eye is too long and so the image focuses
short of the retina
Long sightedness
the eye is too short so the image
focuses past the retina
Correcting short sightedness
a concave lens is used.
Correcting long sightednessa convex lens is used