Biology - Human Eye

Definitions & Structure

Retina

Composed of specialized light sensitive cells or photoreceptors.

Rods

• (low light) response to detect brightness

• located around side of retina

• images are seen in shades of black and white

Cones

• (high light) response to detect color and fine detail.

• located back of retina

• Fovea composed of entirely cones packed together

• detect 3 types: red, green, blue of wavelengths

Accomodation:

• Adjusting the lens shape via ciliary muscles for near and distant vision

Distant Object

• ciliary muscle - relaxes

• sensory ligaments - pulled tightly

• lens - thin

Short distance objects

• ciliary muscles - contracts

• sensory ligaments - slacken

• lens - bulges

Short sightedness (myopia)

• focus only near object

• focus infront of retina (long distance)

• can occur when eyeball is too long

• corrected by wearing diverging (concave) lenses.

Long sightedness (hypermetropia)

• focus only in distant object

• focus behind retina (short distance)

• occurs when eyeball is too short, or lenses is to flat (thin)

• corrected by converging (convex) lenses.

Conjunctiva

Thin, transparent layer continuous with the lining of the eyelids. Protects the cornea.

Cornea

Transparent front part of the sclera. Refracts (bends) light rays onto the retina.

Aqueous Humour

Colourless fluid that maintains the shape of the cornea.

Lens

Transparent, elastic, biconvex structure. Makes fine adjustments to focus light rays on the retina.

Pupil

Hole in the centre of the iris that allows light to enter the eyeball.

Iris

Coloured disc composed of muscle. Controls the amount of light entering the eye.

Suspensory Ligament

Attaches the lens to the ciliary body.

Ciliary Muscle

Circular ring of muscle running through the ciliary body. Alters the shape of the lens during accommodation.

Vitreous Humour

Jelly-like fluid that maintains the shape of the eyeball.

Sclera

Tough, white, fibrous coat that protects the eyeball.

Choroid

Contains blood vessels that supply the retina with food and oxygen and contains melanin to prevent the reflection of light inside the eye.

Fovea

Contains only cones. It is the most sensitive part of the retina, where most light rays are focused.

Retina

Contains light-sensitive cells called rods and cones. This is where the image forms.

Optic Nerve

Carries impulses from the retina to the brain.

Blind Spot

The point where the optic nerve leaves the eye. No light-sensitive cells are present.

Image Formation on Retina

An image of the object is formed on the retina. This stimulates the light-sensitive cells of the retina. The image is inverted, reversed, and smaller than the object.

Optic Nerve and Brain Processing

The optic nerve carries the impulses to the brain. The brain processes the image, turning it the correct way up, in the correct orientation, and forming an impression of its size, shape, colour, and distance.

External Eye Structures:

Cornea, sclera, iris, pupil

Internal Eye Structures:

Lens, retina (with rods and cones), optic nerve

Vision

Focusing of light, conversion of light into electrical signals by photoreceptors

Tear Production

Lubrication and protection of the eye