Download The Human Eye

12.3 – The Human Eye
A beautiful demonstration of physics and nature!
 light rays are refracted (bent) at the eye surface
 fine focusing is done by the crystalline lens
 this lens can accommodate for near and far objects
o near object ? lens made fatter for a short focal length
o far object ? lens made thinner for a long focal length
 accommodation decreases with age
o as we age, the muscles in our eyes don’t work as well
Nearsighted ? aka myopia
 person can best see near
 eye can only focus on nearby objects
 images of far objects are focused in front of retina
 correct with a diverging (concave) lens
Farsighted ? aka hyperopia
 person can best see far away
 eye cannot focus on nearby objects
 images of near objects are focused behind the retina
 correct with a converging (convex) lens
Astigmatism
 eyes are shaped like tin cans or footballs (not round)
 light rays are focused into a line and not to a point
 correct with cylindrical lens
Cataracts
 clouding of the lens of the eye
 a common outcome of old age
 correct with laser eye surgery
o doctor uses a laser to ablate (vaporize/remove) the cloudy
part of the lens and replaces it with a plastic lens
Correcting Vision Using Lenses
Poor vision is often caused by the incorrect _____________ of either the ______________ or
_____________ or the _________________ of the lens. Each condition can be corrected by
__________________ or contact lenses. Some examples of vision problems are __________________,
____________________, and _____________________.
Myopia is _________________________, which means that eyes cannot focus on ______________
objects. The _____________ and the eye’s lens refract the light and bring the rays together. But, if
the eyeball is too ___________, the image forms in front of the retina and when the rays reach the
retina, they have begun to spread out. This makes the image blurry. When a ________________
lens is placed in front of the eye, it spreads out the parallel rays before they reach the eye.
When the eye refracts the light, it is focussed on the ______________.
Hyperopia is _____________________, which means that eyes cannot focus on _____________
objects. Light coming from a nearby object is refracted by the cornea and eye lens, but the
eyeball is too _________. Because of this, the rays reach the retina before they meet. This can be
corrected with a _________________ lens, which bends the rays, bringing them closer together
before they reach the cornea. The eye’s _________ then refracts the rays a little more, and the
rays are focused on the retina.
Presbyopia is the condition in which the eyes’ lenses become ___________ and the ciliary muscles
can no longer make the lenses change _____________. This occurs as a person ages. People
with presbyopia cannot focus on _____________ objects. If someone was already
___________________ and they get presbyopia, they can not focus on distant or nearby objects.
To correct this problem, people wear ______________, which are lenses with two parts. The ______
part of the lens corrects for near-sightedness, and a small ________________ section helps the
eyes focus on nearby objects.
Night-Vision Devices
A night-vision device is an artificial device that allows people to see when a very __________
amount of light is available. The front lens focuses the small amount of light on a plate called a
__________________, which then emits an electron at each point that light hits it.
Electrons are attracted to the next (microchannel) plate. This plate is filled with ______________
of microscopic holes. As the electrons pass through the channels, they collide with the walls and
the walls emit more _______________.
The electrons move to the next plate, which is coated with a ______________. When electrons
collide with a phosphor, the phosphor emits __________ light. The human eye is most sensitive to
green light.
The _______________ focuses the green light onto the eye or transfers the image onto a monitor.
Correcting Vision Using Lenses
Poor vision is often caused by the incorrect shape of either the eyeball or
cornea or the hardening of the lens. Each condition can be corrected by
eyeglasses or contact lenses. Some examples of vision problems are myopia,
hyperopia, and presbyopia.
Myopia
Myopia is near-sightedness, which means that eyes cannot focus on distant
objects. The cornea and the eye’s lens refract the light and bring the rays
together. But, if the eyeball is too long, the image forms in front of the retina and
when the rays reach the retina, they have begun to spread out. This makes the
image blurry. When a diverging lens is placed in front of the eye, it spreads out
the parallel rays before they reach the eye. When the eye refracts the light, it is
focussed on the retina.
Hyperopia
Hyperopia is far-sightendess, which means that eyes cannot focus on nearby
objects. Light coming from a nearby object is refracted by the cornea and eye
lens, but the eyeball is too short. Because of this, the rays reach the retina
before they meet. This can be corrected with a converging lens, which bends
the rays, bringing them closer together before they reach the cornea. The eye’s
lens then refracts the rays a little more, and the rays are focused on the retina.
Presbyopia
Presbyopia is the condition in which the eyes’ lenses become stiff and the ciliary
muscles can no longer make the lenses change shape. This occurs as a person
ages. People with presbyopia cannot focus on nearby objects. If someone was
already near-sighted and they get presbyopia, they can not focus on distant or
nearby objects. To correct this problem, people wear bifocals, which are lenses
with two parts. The top part of the lens corrects for near-sightedness, and a small
bottom/lower section helps the eyes focus on nearby objects.
Night-Vision Devices
A night-vision device is an artificial device that allows people to see when a very
small amount of light is available. The front lens focuses the small amount of
light on a plate called a photocathode, which then emits an electron at each
point that light hits it.
Electrons are attracted to the next (microchannel) plate. This plate is filled with
millions of microscopic holes. As the electrons pass through the channels, they
collide with the walls and the walls emit more electrons.
The electrons move to the next plate, which is coated with a phosphor. When
electrons collide with a phosphor, the phosphor emits green light. The human
eye is most sensitive to green light.
The eyepiece focuses the green light onto the eye or transfers the image onto a
monitor.