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The Human Visual SystemThe Human Visual SystemThe Eye
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Anatomy of the Human EyeAnatomy of the Human Eye
Cornea
Pupil
Iris Sclera
Retina
Optic Nerve
Lens
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Human Visual SystemHuman Visual System
RefractionCornea & Lens
Light ControlIris
(& Pupil)
Light Detection
Retina
ProcessingBrain
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Image FormationImage Formation
The image projected onto the retina is upside down.
Object Image
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CorneaCornea The outer wall of the eye is formed by the hard, white sclera.
The cornea is the clear portion of the sclera.
Most of the refraction takes place at the cornea.Why?
Sclera
Cornea
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Iris and PupilIris and Pupil The colored iris controls the size of the opening (pupil) where the light enters.
The iris controls the amount of light, like the aperture of a camera.
Iris
Pupil
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Iris openDilated pupil
Iris closedConstricted pupil
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LensLens The lens is transparent. The lens can change
shape. This is what allows you to
focus on objects at different distances.
For close objects, the lens becomes thicker.
For far objects, the lens becomes thinner.
Lens
Ciliary muscle
TransparentFibers
Cross section of the eye lens
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AccommodationAccommodation The lens is attached to
the ciliary muscle. When the muscles
contract, the lens becomes thicker.
When the muscles relax, it becomes thinner.
The process by which the lens changes shape to focus is called accommodation.
Relaxed muscleTaut ligamentsDistant object
Near objectContracted muscleSlack ligaments
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Muscle relaxes
Distant object
Lens gets thinner
Object is in focus
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Muscle contracts
Near object
Lens gets thicker
Object is in focus
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Aqueous Humor and Vitreous HumorAqueous Humor and Vitreous Humor
Transparent gelatinous liquid filling the eye.
Provides nutrients to the cornea and eye lens.
Also helps maintain the eyeball shape with its pressure.
Vitreous Humor
Aqueous Humor
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RetinaRetina The Retina is the light detector for the eye.
Two types of receptors in the retina: rods for low light level cones for color.
Signals from the receptors leave through the optic nerve to the brain.
Retina
Optic Nerve
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Blind SpotBlind Spot The retina has a hole
in it where the optic nerves exit the eye.
This is because the nerves run in front of the retina.
Our brain fills in the blank spot with surrounding colour, so it’s hard to notice
“Hole” in retina
Photo receptors
Optic Nerve
Light
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Rods and ConesRods and Cones
Only black, grey, white Only in low light levels.
Rod cell Cone cell
Three types of cones in order to detect colour
Requires a lot of light
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Mach BandsMach Bands
Actual brightness
Perceived by you
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Eye Defects*Eye Defects*
Image focuses on the retina for a normal eye.
Distant objects look blurry for a myopic (near sighted) eye.
Near objects look blurry for a hyperopic (far sighted) eye.
Normal
Myopic
Hyperopic
Object at infinity
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Myopia – Near-sightednessMyopia – Near-sightedness Distant objects look
blurry because the eye cannot relax any farther so that the image is focused before the retina.
Near object in focus without accommodation.
Corrected with a diverging lens.
Myopic eye relaxedBlurry
Myopia corrected with a negative lens
Far object
Near objectMyopic eye relaxedIn focus
Far object
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Hyperopia – Far-sightednessHyperopia – Far-sightedness Near objects look
blurry because the eye cannot accommodate enough for near objects.
Far object in focus. Corrected with a
converging lens.
Hyperopic eye Partially accommodatedIn focus
Hyperopia corrected with a positive lens
Far object
Hyperopic eye Fully accommodatedBlurry
Near object
Near object
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Contact LensContact Lens
Contact lens is an alternative to corrective lenses.
Changes the curvature of the cornea by adhering to the surface with some fluid.
CorneaContact lens
Fluid
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Presbyopia - “Old eye”Presbyopia - “Old eye”
Lens hardens with age. Eye cannot adequately
accommodate near objects.
Bifocals (lens with two focal lengths) contains a concave lens for distance (if needed) and a convex lens for near objects.
Near objects magnified
Far objects
Concave for distance correction (if needed)
Convex for near objectcorrection
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AstigmatismAstigmatism
The cornea is not spherical; Focal length different from one plane to a perpendicular plane.
F’ horizontal
F’ Vertical
Object
Image at F’ Vertical
Image at F’ Horizontal
Cornea
Direction of blur
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AstigmatismAstigmatism
Correction of astigmatism is done through the use of a cylindrical lens.
Cylindrical lens converge rays in one plane but not the perpendicular plane.
Cylindrical lens
Rays in the vertical plane are undeviated
Rays in the horizontal plane are focused
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Common Eye Diseases*Common Eye Diseases*
Cataract - Clouding of the lens. Symptom: Loss of vision Cure: Lens replacement
Glaucoma - Pressure buildup in the eye, damaging the retina. Symptom: Loss of vision first in the periphery. Cure: Surgery to drain fluid from the eye.
Loss of vision is usually permanent
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Common Eye Diseases (cont’d)*Common Eye Diseases (cont’d)*
Detached retina - portion of the retina detaches from the back of the eye. Symptom: Perception of flashes,
Loss of vision Cure: Laser surgery to reattach
retina Pink eye - Infection of the surface of
the eye. Symptom: Irritation Cure: Antibiotics
Ow!
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Your eye careYour eye care
Go see a doctor if you think there is something wrong with your eye-
Early detection is essential to keeping damage low and preventing permanent loss of your vision.