THE HUMAN EYE

Section - 13.6 PG. 572 - 577

PARTS OF THE HUMAN EYE

PARTS OF THEHUMAN EYE

Iris – coloured part of your eye, it opens and closes around a central hole to let in more or less light

Pupil – hole in the iris, where light enters the eye

Lens – structure that causes light entering the eye to converge

Cornea- tissue structure that causes light to converge as well, it is the bulge on top of the pupil that focuses the light

Light is refracted more through the cornea than through the lens

PARTS OF THE HUMAN EYE

Retina – converts the light signal into an electrical signal that is transmitted to the brain through the optic nerve

Optic Nerve – creates a blind spot at the back of each eye because there are no light sensitive cells in that small area. The blind spot goes unnoticed because each eye compensates for the blind spot of the other eye.

EX: Left eye can see what is in the blind spot of your right eye and vice versa

THE LIGHT GATHERING

INSTRUMENT

THE LIGHT GATHERING

INSTRUMENTThe eye acts a light gathering instrument for the brain

The lens-cornea combination of the eye acts like a converging lens and produces a smaller, real, inverted image on the retina

Electrical impulses from the retina travel through the optic nerve to the brain

The brain takes the inverted image that came from the retina and flips it so the image we see is upright

ACCOMMODATION

& FOCUSAccommodation – changing of shape of the eye lens by eye muscles to allow a sharply focus image to form on the retina

Shape of the eye lens changes in order to allow an inverted focused image to form on the retina

Ciliary muscles help focus the eye on distant and nearby objects by slightly changing the shape of the eye lens

Eye lens changes the focal lengths of the lens to allow focusing of the image on the retina

Not all people have properly functioning ciliary muscles which results in focusing problems

FOCUSING PROBLEMS

Hyperopia (Far-sightedness) – the inability of the eye to focus light from near objects

No issues seeing distant objects, issues seeing nearby objectsThe eye cannot refract light well enough to form an image on the retinaOccurs when the distance between the lens and retina is too small or when the lens-cornea combination is too weakLight ends up focusing behind the retina resulting in a blurred image

HYPEROPIA

FOCUSING PROBLEMSPresbyopia – a form of far-sightedness caused by a

loss of accommodation as a person agesCondition when the eye loses elasticity, usually an age related vision issueCan be corrected by glasses or contacts with a converging lens shape

Myopia (Near-sightedness) – the inability of the eye to focus light from distant objects

The eye can focus light rays from nearby objects allowing them to see up close clearly, however issues with seeing distant objectsOccurs when the distance between the lens and retina is too large or because the cornea-lens combination is too strongLight from distant objects is brought to a focus in front of the retina

MYOPIA

PRESBYOPIA

CORRECTING VISION

PROBLEMSCorrecting HyperopiaA converging lens will correct far-sightedness & a lens with a positive meniscus has the same effectPositive meniscus lens is a modified form of the converging lens shape (middle part thicker than the edges)

Correcting MyopiaA diverging lens will correct near-sightedness & a lens with a negative meniscus has the same effectNegative meniscus lens is a modified form of the diverging lens shape (edges are thicker than the middle part)

CORRECTING VISION

PROBLEMS

CORRECTING VISION

PROBLEMSContact LensesA lens that is placed directly on the cornea of the eyeServe the same purposes as glasses, can be shaped to correct both far and near-sightednessCan be used for cosmetic purposes; ex. Changing the colour of the eyes

IN SUMMARYThe cornea-lens combination in the eye acts like a converging lens; the brain flips the inverted image that is receives from the eye so that what you see is upright

The eye focuses through accommodation; the shape of the eye lens is changed slightly by eye muscles

Hyperopia means that a person is far-sighted; near vision corrected with a converging lens

Presbyopia is an age-related condition of far-sightedness that is caused by a loss of accommodation

Myopia refers to a person who is near-sighted; distance vision is corrected with a diverging lens