By By Dr. Abdelaziz Hussein Dr. Abdelaziz Hussein

Ò Light is a form of radiant energy, consisting of electromagnetic waves

a. Velocity of light: In air it is 300,000 km/second. b. Wave length: The wave-length of visible light to the human eye ranges from 400

to 750 nm. The ultraviolet (less than 400 nm) and infrared radiations (more

than 7500 nm).

c. Reflection and Refraction: Ò When light rays strike a surface, they are A. Reflected ( white objects reflect all light) B. Absorbed (black objects absorb all rays) C. Transmitted through it, with or without refraction as when pass

through lenses

1) Convex Lens: The parallel rays passing

through the convex lens converge to one point called the focal point or focus.

2) Concave Lens: The light rays passing through

the concave lens diverge. Thus, the concave lens diverges light rays, but the convex lens converges light rays.

1) Spherical Lens: It is a part of sphere. It converge or diverge light

into one point It is used for correction of

myopia and hypermetropia

2) Cylindrical Lens: It is a part of cylinder It converge or diverge light

into one plane It is used for correction of

astigmatism

Velocity of light in air Velocity of light in air Refractive index Refractive index = = 1.0 for air = = 1.0 for air

Velocity of light in medium Velocity of light in medium 1 1

Refractive power Refractive power = = diopter = = diopters s Focal length in meters ( Focal length in meters (distance between nodal point 

and focal point)

The eyeball wall consists of 3 layers

Ò 4 refractive media in the eyeball 

•Cornea→ constant (unchanging) refraction •Lens→ can change refraction and focal length by ciliary ms

Ò The cornea is the anterior transparent 1/6 of outer layer of wall of eyeball

Structure: Structure: 1. Stratified squamous epithelium. 2. Basement membrane 3. Substantia propria formed of

bundles of collagen fibers and matrix.

4. Descment membrane 5. Endothelium

The normal cornea is transparent to allow passage of light and formation of sharp clear image on retina.

Causes of transparent cornea: Causes of transparent cornea: 1) Anatomical factors: i. Regular arrangement of epithelium ii. Regular arrangement of collagen fibers in substantia propria iii. Absence of blood vessels (avascular) and myelinated nerve

fibers. 2) Relative corneal dehydration: By metabolic pump (in endothelium) that pump Na ions into the

aqueous humour →↑ed osmolarity of it → osmosis of water out of the cornea.

3) Refractive index of collagen fibers, matrix, and epithelium is the same

Factors that maintain corneal transparency: 1) Vit A: -Essential for maintenance of healthy epithelium. -Its deficiency causes keratinization of cornea. 2) Vit B2: -Essential for normal respiration of avascular cornea. -Its deficiency causes hypoxia of cornea and its vascularization. 3) Moistening of cornea by precorneal tear film 4) Metabolic pump: -

1) Refractive medium: Ò -Acts as convex lens of +39 diopters. Ò -The most important refractive medium of eye (70% of

refractive power of the eye). 2) Transparent: Ò -allows passage of light to inside eye and formation of sharp

clear image on retina 3) Regular curvature: Ò Help the formation of sharp and clear image on the retina 4) Protection of eye: by Ò Absorb UV rays Ò Tough fibrous coat Ò Corneal reflex

1) Corneal opacity Loss of corneal transparency. 2) Keratoconus (Conical cornea): Conical protrusion of cornea due to

congenital weakness. Treated by contact lenses or corneal

grafting. 3) Astigmatism: Abnormality in which the curvature of the

cornea is not the same in all meridians. Treated by cylindrical lenses

Ò Transparent colorless alkaline fluid.

Ò It fills anterior and posterior chambers of the eye.

Ò It is formed continuously by the ciliary epithelium by diffusion and active transport and drained by canal of Schlemm at corneo-scleral junction

Ò Rate: 1-2 ul/min

1. Refractive medium, its refractive index is 1.33.

2. Nourishment of the avascular cornea and lens

3. Buffers the acid produced by anaerobic metabolism of the cornea and lens.

4. Keeps the eyes rigid and maintains its refractory power

5. Maintains intraocular pressure constant by its steady formation and drainage.

Ò  It is an elastic biconvex, transparent circular lens. 

Ò  It is about 11 mm in diameter. Ò  It is suspended to the ciliary body by the suspensory ligament. 

Ò  It has a refractive power of 20 diopters during rest, but its power increase during accomodation to near vision 

Ò  It has a refractive index of about 1.40

Ò Aphakia → Absence of the lens from the eye Ò Treated by 2 pair of glasses Ò Cataract→ It is the loss of lens transparency Ò Treated by surgical removal of cataract and

intraocular lens Ò Presbyopia →old sight Ò Errors of refraction →myopia, hypermetropia

and astigmatism

1. Refractive medium, its refractive index is 1.34.

2. Supports the retina.

3. Supports the lens. 4. Maintains the

spherical shape of the eye.

Ò The eye function as a camera. Ò Light rays falling on the eye converge to a focus on

the retina. Ò The retina is similar to the sensitive film of the

camera. Ò The pupil regulates the amount of rays that enter

the eye, like the diaphragm of the camera.

Ò These are inverted images. Ò The size of the image can be calculated if the size of

the object and its distance from the eye (in meters) are known, as follow:

Size of object = Distance of object Size of image Distance of image from lens

Ò  In the resting normal eye, parallel rays from distant objects are focused on the retina. 

Ò The object distance from the eye is considered infinity, and the distance of the image on the retina from the lens is about 17 mm (0.017 meter), therefore the diopteric power of the resting eye can calculated from the following formula:

1/object distance + 1/image distance = 1/f = power of the lens

1/infinity + 1/0.017 = zero + 1/0.017 =1/f =  59 diopters

This is a process by which the optical system of the eye is adjusted to see near objects clearly. 

Eye changes during accommodation: 

1­ Constriction of pupils (Miosis) → due to contraction of constrictor pupillae ms 

2­ Bilateral increase thickness of the lens→ due to contraction of ciliary ms 

3­ Bilateral medial convergence → due to contraction of both medial rectii

•Def: It is the distance between the far point of distinct vision (Normally infinity) and the near point of distinct vision 

•Far point of vision It is the distance beyond which lens will not change its shape (about 20 feet or 6 meters) (flattest point of the lens) 

•Near point: It is the shortest distance at which the eye can see clearly & at which the accommodation is maximum. 

It recedes by aging due to the decrease of the lens elasticity & ciliary muscle power.

•Def: Ò  It is the difference in refractive power of lens when the accommodation maximum (at near point) and when it is zero (at far point). 

Calculation of amplitude of accommodation: In young adults, the near point for clear vision is about 10 cm, so 1  +  1  =  1  = 69 diopters 0.10      0.017      f 

So amplitude of accommodation is 69­59= 10 diopters

Amplitude of

accommodation

Near point( cm ) Age ( In years )

14.0 7 10

10.0 10 20

7.0 14 30

4.5 22 40

1.0 100 60

This is a physiological condition of decreased power of accommodation that occurs at the age of about 45 years.

It is due to gradual loss of lens elasticity. The near point of clear vision recedes far so that reading

and other close works become difficult. It is corrected by convex lenses

Emmetropic (Normal) eye: It is the eye in which parallel rays converge to a

focus on the retina Ammetropic eye: It is the eye in which parallel rays can not converge

to a focus on the retina

ERRORS OF REFRACTION ERRORS OF REFRACTION

An optical eye defects in which parallel rays striking the resting eye do not form a focus on the retina. 

They include: 1­Hypermetropia or Hyperopia 2­Myopia 3­ Astigmatism

Ò  The parallel rays striking the resting eye form a focus behind the retina. 

Ò  It is due to either short eye ball or weak optical system of the eye 

Ò  The patient complains of headache and eye strain due to continuous contraction of the ciliary muscle 

Ò  It is corrected by biconvex lenses

Ò  This is an optical eye defect in which parallel rays striking the resting eye form a focus in front of the retina producing a blurred vision. 

Ò  It is due to abnormally long eyeball or abnormally strong optical system. 

Ò  It is corrected by concave lenses.

It is due to an abnormal irregular curvature of the anterior surface of the cornea and less commonly of the lens. 

It is a congenital condition in most cases. However it may be acquired following inflammation of injury of cornea. 

It is corrected by cylindrical lenses.

THANK YOU