The Human Eye

The human eye is similar to a camera!!Light enters through an opening, is focused through a lens,

passes through a light-tight (dark) space, and forms a smaller, inverted, real image on a light-sensitive surface at the back.

Parts of the Human Eye

Cornea

(clear covering)

Iris (coloured disc)

Pupil

Pupil

Cornea

Iris

Lens

Ciliary MuscleVitreous Humour

Fovea

Retina

OpticNerve

Sclera

Sclera

Path of Light into the Eye

CorneaTransparent, outer layer of eye.Refracts light towards pupil.

PupilBlack hole in the iris.Allows light to pass through to the lens.

IrisCircular band of muscle and pigment.Controls the size of the centre hole (contracts or dilates the pupil).

Focusing the Light-Lens

Transparent, flexible, crystalline convex lens behind pupil.

Refracts and focuses (converges) light.

-Ciliary MuscleControls shape of lens to

allow lens to focus.

Support Structure of Eye

• Sclera – White, dense outer layer of

eye ball. For eye muscle attachment, holding eye shape, and to keep inside of eye dark.

• Vitreous humour– Transparent fluid gel that holds

the shape of eye; allows light to pass from lens to retina.

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Image Formation in the Eye• Retina

– Inner lining of light-sensitive cells that covers back of eye; surrounded by network of nerves.

– Detects light (of the real image), and transmits this as a signal to the optic nerve.

– Photoreceptive cells in the retina are referred to either as “rods” or “cones”.– Rod cells are more sensitive to light

(therefore details).

– Cone cells detect either red, green, or blue wavelengths of light. (therefore colour vision).

About 10% of males have a colour perception defect, but this is rare in females.

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• Fovea– Area of retina which contains the highest concentration

of photoreceptive cells.

Optic Nerve– Bundle of all the nerves from the retina, located at the

back of the eye ball. Transmits all the optic signals to the brain for image processing.

Image Formation in the Eye

Accommodation

• The distance between the lens and the image (di) is constant in the eye.

• However, the eye can focus on objects far away or very close to the eye.

• How is this possible?

AccommodationAccommodation: the ability of the eye to change the focus between objects at different distances by altering the curvature of the lens.

What changes

the shape of the lens

in our eyes?

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Accommodation

• When the ciliary muscles contract, the lens is stretched so that less refraction takes place.

• This allows us to see objects that are far.

• When the ciliary muscles relax, the lens thickens so that more refraction takes place.

• This allows us to see objects that are near.

Note: This is UNLIKE a camera, where the lens is moved forward or backward to focus.

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Accommodation Not Perfect

• Either by heredity or age, accommodation of the human eye can be flawed.

• In general, people can either suffer from Myopia (near-sightedness) or Hyperopia (far-sightedness). People can also suffer from Astigmatism or develop Presbyopia.

Normal Eye Focus

Near-Sighted (Myopia)

Far-Sighted (Hypermetropia)

Astigmatism• Cornea is oval shaped instead of spherical.

This causes light to focus on two or more spots.

Fixing Eye Problems

1. Glasses

2. Contacts

3. Surgery

1. Glasses for Near-Sighted (Myopia) People

Use a Concave Lens

1. Glasses forFar-Sighted (Hypermetropia) People

Use a Convex Lens

2. How a contact lens works2. How a contact lens works• Contact lens works just Contact lens works just

like a pair of glasses, it like a pair of glasses, it bends light in a specific bends light in a specific direction in order to have direction in order to have a correct light projected on a correct light projected on the retina. the retina.

Disease caused by wearing the contact Disease caused by wearing the contact lenses improperlylenses improperly

• Corneal Ulcer -Corneal Ulcer -Bacterial infections cause corneal Bacterial infections cause corneal ulcers and are common in people who wear contact ulcers and are common in people who wear contact

lenseslenses

It is a very serious disease and it only can be cured by the cornea transplantation.

Giant Papillary Conjunctivitis Giant Papillary Conjunctivitis -It is -It is caused by the uncleanness of the lenses. caused by the uncleanness of the lenses.

Allergy Allergy -from the cleaning agent of the lenses-from the cleaning agent of the lenses

• 7. Conjunctivitis 7. Conjunctivitis -It is caused by the invasion of the -It is caused by the invasion of the bacteria, it will caused the inflammation of bacteria, it will caused the inflammation of the cornea and it is an infectious disease.the cornea and it is an infectious disease.

Laser Eye Surgery

2. What is LASIK?

• Procedure uses computer controlled laser to reshape cornea

• Surgical procedure to correct – Myopia– Hyperopia– Astigmatism

• LASIK is acronym for laser assisted in situ keratomileusis

• Slits are cut in the eye to correct myopia (decreasing curvature) or astigmatism.

Video on Laser Eye Surgery

• http://www.mayoclinic.org/procedure/lasik-eye-surgery/multimedia/lasik-eye-surgery/vid-20084660