The physics of vision
• The eye is essentially an
opaque eyeball filled
with a water-like fluid.
• In the front of the eyeball
is a transparent opening
known as the cornea, a
thin membrane with an
index of refraction of
approximately 1.38. The
cornea has the dual
purpose of protecting the
eye and refracting light
as it enters the eye.
The physics of vision
• After light passes
through the cornea, a
portion of it passes
through an opening
known as the pupil,
which is merely an
opening. The pupil is the
black portion; the size of
the pupil opening can be
adjusted by the dilation
of the iris. The iris is the
colored part of the eye
The physics of vision
• Light then enters the
crystalline lens. The
crystalline lens has an index
of refraction of roughly
1.40. The lens is attached to
the ciliary muscles. These
muscles relax and contract
in order to change the
shape of the lens. By
carefully adjusting the
lenses shape, the ciliary
muscles assist the eye in
the critical task of
producing an image on the
back of the eyeball.
The physics of vision
• The inner surface of the eye is
known as the retina. The retina
contains the rods and cones. An
adult eye is typically equipped
with up to 120 million rods that
detect the intensity of light and
about 6 million cones that detect
the frequency of light. These rods
and cones send nerve impulses
to the brain. The nerve impulses
travel through a network of nerve
cells. There are as many as one
million neural pathways from the
rods and cones to the brain. This
network of nerve cells is bundled
together to form the optic nerve
on the very back of the eyeball.
The physics of vision
• The cornea and lens serve to refract light
and focus an image of the object on the
retina
• The cornea-lens system has a focal length
of about 1.8cm.
The physics of vision
• the cornea-lens system is able to change
its focal length. The ciliary muscles
contract and relax, changing the shape of
the lens. This allows the eye to change its
focal length and focus images of objects
that are both close up and far away.
The physics of vision
• the lens of the eye assumes a large curvature
(short focal length) to bring nearby objects
into focus and a flatter shape (long focal
length) to bring a distant object into focus.
Vision problems usually arise when the eye is
unable to change the shape of the lens and
the image is no longer focused on the retina.
Accurate vision
Nearsighted Correction
Far sighted correction
Homework
• To form an image, light passes through FIVE different
media [including air] in the eye, each with a different
index of refraction. Draw a cross sectional view of the
eye and label each of the 5 media and the retina. If
incident light strikes the eye at 10⁰, then travels through
each of the media, at what angle (from the normal!) will
it strike the retina at the back of the eye?
• nair = 1.00
• n2 = 1.38 (cornea)
• n3 = 1.33 (aqueous humor)
• n4 = 1.4 (lens)
• n5 = 1.34 (vitreous humor)
• Retina