VISION How do we see?

Why do we cry? Near the top and outer side of the eye, the Lacrimal glad makes tears to wash across the eye. Tears drain into the airway of the nose, so our nose runs when we cry!

PUPIL Light enters the eye through the pupil, which appears black because light travels through it and does not bounce back.

IRIS The colorful circle of muscle surrounding the pupil is called the iris.

SCLERA It is the white of the eye. It is opaque, fibrous, and protective. In other animals, it is not necessarily white, but the same color as the iris.

CORNEA Covering the iris and pupil is a transparent tissue called the cornea.

LENS Behind the pupil is a flexible lens, that can get thicker or thinner to focus on objects.

RETINA Light rays pass through the lens and are focused on the back of the eye on the retina. They converge on the fovea.

VITREOUS HUMOR Is the clear gel that fills the space between the lens and the retina of the eyeball in humans and other vertebrates. It is transparent and colorless and gives the eye shape.

AQUEOUS HUMOR This is a SMALLER gel-like humor that is found between the lens and the cornea.

IMAGES ARE INVERTED! Light rays on the top of an object hit the bottom of the retina. Light rays on the bottom of an object hit the top of the retina. So the image is inverted!!!

OPTIC NERVE Information is sent from the retina by electrical signals, through the optic nerve to the brain. The brain turns the image the right way up again!!

More about the RETINA RODS CONES GANGLION CELLS On the retina, we must have special cells that can sense light rays. These are called photoreceptors. RODS -sense black and white, and dim light CONES -sense colors and daytime light GANGLION CELLS -reflexive response to bright daylight

RODS Specialized cell on retina for detecting less intense light Found in the periphery of the eye 125 million rod cells in the human retina Responsible for night vision

CONES Responsible for color vision, function best in bright light Densely packed in the fovea 4.5 million cone cells in human retina Perceive finer details and are faster perception than rods

RODS AND CONES Cones sense red, blue, and green light waves

CAN YOU NAME THE PARTS?

CORNEA-LENS-RETINA SYSTEM Light rays enter the cornea, which does MOST of the focusing. They converge after they enter the cornea.

The LENS does the rest of the focusing Muscles make the lens THICKER to focus on near objects. Muscles make the lens THINNER to focus on far objects.

LENS Problems! If a lens is not the right size, or does not become thick or thin enough, images will be blurry!

Nearsightedness If we can see near, but not far, we are NEARSIGHTED. Basically, our lens does not become thin enough to focus on far objects. We can correct this with CONCAVE lens

Farsightedness If we can see far, but not close up, we are FARSIGHTED Basically, our lens does not get thick enough to see things up close. This can be corrected with a CONVEX lens.

BLIND SPOT This is where the optic nerve enters the brain There are NO photoreceptor cells here to detect light! So we can not see here!

BLINDNESS

SNOW BLINDNESS

NIGHT BLINDNESS

COLOR BLINDNESS

GLASSES

CONTACT LENSES

LASIK SURGERY

Photorefractive Keratectomy (PRK)