Transcript 12.3 - Lens Technologies and The Human Eye

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Cornea:
◦ Tissue that forms a transparent, curved structure in
front of the eye
◦ Refracts light before it enters the eye
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Retina:
◦ A layer of cells (known as rods and cones) that
respond to light and initiate nerve responses
 Rod cells are very sensitive to light but cannot
distinguish between colours
 Cone cells detect colour
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Recall: normally, when an object moves in front of a lens,
its image moves; moving the lens has the same effect
This won’t work in the eye – images from different
distances need to focus on the retina, but the
distance between the retina and the lens is always
the same
So what happens?
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The cornea refracts light in the same way
regardless of the location of the object
The lens, however, can change shape and refract
light to a different extent
This allows it to focus light from both nearby and
distant objects onto the retina
The ciliary muscles (or ciliary bodies) make the lens
shorter and thicker by pulling on it
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This process is called accomodation
To focus on a nearby object, the curvature of the
lens needs to be greater because it needs to refract
light more
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Cameras are designed very much like eyes
Both have lenses that focus light on a
light-sensitive material
◦ Retina in the eye
◦ Film or CCD in a camera
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Lens of the eye changes shape in order to
focus on objects at different distances
Lens of the camera must be moved in and
out to focus on objects at different
distances
Eye has an iris (or pupil) that controls the
amount of light entering it
Camera also has an iris (or aperture) to
control the amount of light
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Myopia is near-sightedness
◦ The eyes cannot focus on distant objects
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Caused by the eyeball being too long
The image forms in front of the retina, not on it
By the time light rays reach the retina they have
begun to diverge, and the image looks fuzzy
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Adding a diverging lens spreads out the parallel light
rays before they reach the eye
The rays that are separating from each other appear
to be coming from an object that is closer to the eye
Due to the higher angle of incidence, when the eye
refracts the light, it is focused correctly on the retina
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Hyperopia is far-sightedness
◦ The eyes cannot focus on nearby objects
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Caused by the eyeball being too short
Light rays entering the eye reach the retina before
they converge at a point, causing the image to be
blurry
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Adding a converging lens bends the rays slightly
inward before they enter the eye
The lens of the eye then refracts them more, and
causes the rays to be correctly focused on the
retina
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As a person ages, the lenses of the eye become stiffer, and
the ciliary muscles can no longer make them change shape
This condition is called presbyopia
People with presbyopia cannot focus on nearby objects, but
this is not farsightedness (which is caused by an incorrect
eyeball length)
If someone is already nearsighted and gets presbyopia, they
cannot focus on distant OR nearby objects!
To correct this condition, people wear bifocals
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Bifocals are lenses with two parts
◦ The top part of the lens corrects for nearsightedness
◦ A small section of the lower part with a different curvature
helps the eye focus on nearby objects
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Bifocal contact lenses are also available
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Astigmatism is blurred or distorted vision that is
usually caused by an incorrectly shaped cornea
Instead of being rounded, the cornea is oval-shaped
Part of an image might be in focus, but the rest of
the image is blurry
This is corrected by using lenses that have a
cylindrical curvature
◦ Refracts incident light rays along one axis only