The iris is the coloured circle of muscle surrounding the pupil. 6.1 Human Vision Parts of the Eye The pupil is the dark transparent region in the centre of the eye where light enters. The iris is the coloured circle of muscle surrounding the pupil. The iris controls the amount of light entering the eye. The sclera is the white part of the eye surrounding the iris. See pages 202 - 203 (c) McGraw Hill Ryerson 2007

The cornea is the transparent tissue covering the iris and the pupil. 6.1 Human Vision Parts of the Eye b The cornea is the transparent tissue covering the iris and the pupil. Behind the pupil is a flexible convex lens. The lens focuses light onto the retina located in back of the eye. The retina is covered with light sensitive cells that convert light energy into electrical energy. Electrical signals are sent to the brain by the optic nerve. a c d Locate the cornea, optic nerve, lens, and retina. See pages 202 - 203 (c) McGraw Hill Ryerson 2007

The Cornea-Lens-Retina System Light rays first entering the eye are refracted by the cornea so that they converge toward the retina. Light then passes through the lens which “fine-tunes” the focus. The image that forms on the retina is inverted. The area where the optic nerve enters the retina is called the blind spot. This area has no light-sensing cells. See pages 204 - 205 (c) McGraw Hill Ryerson 2007

Black-and-White Vision and Colour Vision The retina contains two types of light-sensitive cells called rods and cones. Rod cells are cylinder-shaped cells that allow us to see images in shades of light and dark when the light is dim. Cone cells are cone-shaped cells that allow us to see colour in bright light. rod cone Electron micrograph of the retina. See page 206 (c) McGraw Hill Ryerson 2007

Correcting Focus Problems Near-sighted vision Can not clearly focus on distant objects. Occurs because the lens converges the light rays to form an image in front of the retina. A concave lens is used to correct near-sighted vision. See pages 208 - 209 (c) McGraw Hill Ryerson 2007

Correcting Focus Problems Far-sighted vision Can not clearly focus on nearby objects. Occurs because the lens converges the light rays to form an image behind the retina. A convex lens is used to correct far-sighted vision. See pages 208 - 209 (c) McGraw Hill Ryerson 2007

Correcting Focus Problems Astigmatism Blurred vision due to a irregular shaped cornea. Causes the image to focus on more than one point on the retina. Corrected by using eyeglasses, contact lenses, or laser surgery. See pages 208 - 209 (c) McGraw Hill Ryerson 2007

Most people who are legally blind can perceive some light. Blindness Blindness is any vision impairment that keeps people from carrying out important life functions. Most people who are legally blind can perceive some light. May be able to see a tiny part of the middle of the whole scene (tunnel vision). May be able to only see the edges but not directly ahead. May be able to see light and dark but not clearly, even with visual aids. Normal vision Tunnel vision See page 209 (c) McGraw Hill Ryerson 2007

Other Types of Blindness Snow blindness is a temporary blindness caused by overexposure to the glare of sunlight. Night blindness is a condition in which it is difficult to see in dim light. Colour blindness is the ability to see only in shades of grey. Colour vision deficiency is the inability to distinguish certain colours. The most common is the inability to distinguish between red and green. A test for red-green colour vision deficiency. Take the Section 6.1 Quiz See page 210 (c) McGraw Hill Ryerson 2007