1. Human Vision

2. 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.

3. 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.

4. 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.

5. 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. cone rod Electron micrograph of the retina.

6. 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

7. 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.