The Eye and Sight Describe the relationship between the structure of the eye and sight Contrast ways in which light rays are bent by concave and convex lenses. Identify types of corrective lenses used to correct different sight problems (Example: convex – farsightedness, concave – nearsightedness)

http://www.aoa.org/x6024.xml http://www.tpub.com/neets/tm/106-3.htm Vision begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye. cont

CORNEA The cornea bends or refracts the rays that pass through a round hole called the pupil.

IRIS The iris, or colored portion of the eye that surrounds the pupil, opens and closes.

PUPIL The pupil gets bigger or smaller to regulate the amount of light passing through.

LENS The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye.

Structure of the Eye: Cornea and Lens Retina Cornea Eyelens There are two lenses in your eye, the cornea and the eyelens. The cornea, the front surface of the eye, does most of the focusing in your eye The eyelens provides adjustable fine-tuning of the focus

The Eyelens: Accommodation The eyelens changes its focal length by changing its shape. Ligaments pull on the lens to change the amount of “bulge”

Eyelens: Accommodation Muscles contract, ligaments relax, more bulge, more bending power, shorter focal length Ligaments Eyelens Muscles relax, ligaments contract, less bulge, less bending power, longer focal length

RETINA The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells. The images that we see are projected onto the retina upside down.  Our brain quite simply, flips the images over so that we see things upright.

Structure of the Eye: Retina The retina is the sensor or film of your eye. Its layers do three things Provide blood and nutrients (choroid) Absorb light and convert to an electrical signal (photoreceptors) Transfer the signal to the brain (nerve cells) Light Plexiform layer (nerve cells) Rods and Cones (photoreceptors) Choroid (blood vessels)

Structure of the Retina Light ← Nerve cells ← Photoreceptors ← Choroid

Photoreceptors: Rods and Cones Light is detected and converted to an electrical signal by the photoreceptors in the retina. There are two main kinds of receptors, rods and cones This is a false color image, rods and cones are not actually different colors rod cone

Photoreceptors: Cones Cones are responsible for our fine detailed and color vision Cones are clustered near the center of your retina, called the fovea There are 5 million cones in the average retina

Photoreceptors: Rods Rods are responsible for low light and peripheral vision They are present everywhere in the retina except the fovea There are 125 million rods in the average retina

Rods and Cones Because of their different functions, rods and cones are present in varying densities in the retina. The blind spot is due to the connection of the optic nerve

The optic nerve transmits information to the brain.

The vitreous body, or macula, gives the eye its shape.

What are lenses? Lenses bend light in useful ways. Most devices that control light have one or more lenses in them (some use only mirrors, which can do most of the same things that lenses can do). There are TWO basic simple lens types: Concave and Convex

CONVEX or POSITIVE lenses will CONVERGE or FOCUS light and can form an IMAGE. http://www.passmyexams.co.uk/GCSE/physics/use-of-lenses-for-correcting-vision-eyesight.html By wearing a convex (converging) spectacle lens, the rays of light from a near object are converged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.

Convex lens The correct name for farsightedness is Hyperopia. The shape of your eye does not bend light correctly, resulting in a blurred image. A convex lens is usually used to correct this problem.

CONCAVE or NEGATIVE lenses will DIVERGE (spread out) light rays. http://www.passmyexams.co.uk/GCSE/physics/use-of-lenses-for-correcting-vision-eyesight.html By wearing a concave (diverging) spectacle lens, the rays of light from a near object are diverged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.

Concave lens The correct name of nearsightedness is Myopia. Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. A concave lens is usually used to correct this problem.

http://www. sciencelearn. org http://www.sciencelearn.org.nz/Contexts/Exploring-with-Microscopes/Sci-Media/Images/How-lenses-magnify In simple magnification, light from an object passes through a biconvex lens and is bent (refracted) towards your eye. It makes it appear to have come from a much bigger object.