The Human Eye13.6 Optics

THE HUMAN EYE:How Images Are Formed• The human eye gathers light from objects.• In a healthy eye, a smaller, inverted, real image of an

object is created on the retina at the back of the eye.• Electrical impulses from the eye travel through the

optic nerve to the brain.

13.6

The brain• Takes inverted image from the retina

and flips it so that the image we “see” appears upright

In a healthy eye, a smaller, inverted, real image of an object iscreated on the retina at the back of the eye.

• Eye muscles called ciliary muscles slightly change the shape of the eye lens.

• The process of accommodation makes it possible to create a sharply focused image on the retina if an object is distant or nearby.

Parts of the Human Eye

Anatomy of the Eye

Iris• Coloured part of eye• Opens and closes around a central hole to controls

the amount of light that gets in

Pupil• The hole in the iris

Parts of the Human Eye

Cornea & Lens combination• Cornea – transparent bulge over the lens• Together, acts like a converging lens• Produces a smaller, real, inverted image on the retina

Parts of the Human Eye

Retina• Light sensitive cells in the retina convert light signals

into electrical signal that is transmitted to the brain through the optic nerve

Optic Nerve• Creates blind spot at the back of each eye

(but is compensated for by the other eye)

Parts of the Human Eye

Anatomy of the Eye

Perceiving and Seeing• The retina is an extension of the brain and consists of several

complex layers of nerve cells. • Light from different directions are received by different parts of

the retina because the retina is not perfectly consistent in terms of the sharpness of the image we perceive.

• The fovea is the are in the center of the field of view which produces the clearest image.

• There is also something called the blind spot in the eye and this is where the nerves carrying all the information leave the eye in a narrow bundle.

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Perceiving vs. seeing in the brain• The physical task of forming an image is

only one part of the human perception of light.

• In order to see, the light entering your eyes must be absorbed by photoreceptors, light sensitive cells found in the retina that are shaped as rods or cones.

• Rod cells allow for the detection of shapes and movement in low light and only shades of grey are detected by these cells.

• Cone cells are the photoreceptors that detect three primary colours: red, green, or blue.

Accommodation

• Our ciliary (eye) muscles help the eye focus on distant and nearby objects by slightly changing the shape of the eye lens, thereby changing the focal length to allowing the eye to focus the image on the retina

• The process of accommodation makes it possible to create a sharply focused image on the retina if an object is distant or nearby.

Focusing Problems

Hyperopia (far-sightedness)• Can see far / can not see nearby objects• Cause:– distance between lens and retina too small, or– cornea-lens combination too weak

Hyperopia

Hyperopia (far-sightedness)

• Result: light from all nearby objects focuses behind the retina

Hyperopia (far-sightedness)

• Solution: – Far-sighted eyes need help refracting light– a corrective converging lens (with a positive

meniscus)

Focusing Problems

Presbyopia• A form of far-sightedness caused by a loss

accommodation as a person ages• Eye lens just loses its elasticity

Focusing Problems

Myopia (near-sightedness)• Can not see far / can see nearby objects• Cause:– Distance between lens and retina is too large, or– Cornea-lens combination converges light too

strongly

Myopia

Focusing Problems

• Result: light from distant objects is brought to focus in front of the retina

Focusing Problems

• Solution: a corrective diverging lens (with a negative meniscus)

Focusing Problems

Astigmatism• abnormal curvature of the cornea (oval shape) can

cause two focal points to fall in two different locations making objects up close and at a distance appear blurry

Astigmatism

Focusing Problems

Glaucoma• the eye’s drainage system becomes clogged so the

intraocular fluid cannot drain. As the fluid builds up, it causes pressure to build within the eye. High pressure damages the sensitive optic nerve and results in vision loss.

Focusing Problems

Cataracts• A clouding of the eye’s natural lens • Blocks light from reaching the retina and interferes

with vision

• Cataract Surgery

Eye Floaters (Savanna!)

• Eye floaters are small moving spots that appear in your field of vision. They may be especially noticeable when you look at something bright, such as white paper or a blue sky.

• Most of the time people learn to live with eye floaters and ignore them. And they often improve over months to years. Only rarely do benign eye floaters become bothersome enough to consider treatment.

• But sometimes eye floaters are a sign of a more serious condition. You should seek immediate medical attention if you notice a sudden increase in the number of eye floaters.

• Eye floaters move as the eyes move. They generally appear to dart away when you try to focus on them.

• Eye floaters can appear in many different shapes, such as:– Black or gray dots– Squiggly lines– Threadlike strands, which can be knobby and semi-transparent– Cobwebs– Ring shaped

Causes of Eye Floaters• Most eye floaters are caused by small flecks of a

protein called collagen.• The back compartment of the eye is filled with a

gel-like substance called vitreous humor.• As you age, the vitreous and its millions of fine

collagen fibers shrink and become shred-like. Shreds can accumulate in the vitreous. This can cause a change in the amount of light that hits the These changes can happen at any age. They most often occur between ages 50 and 75, especially in people who are very nearsighted or have had cataract surgery.

Eye Colour

What Eye Colour will you Children Have?• http://

genetics.thetech.org/online-exhibits/what-color-eyes-will-your-children-have

Eye Colour• http://www.eyedoctorguide.com/eye_general/eye_color_genetics.html