(c) mcgraw hill ryerson 2007 6.1 human vision the pupil is the dark transparent region in the centre...
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(c) McGraw Hill Ryerson 2007
6.1 Human Vision
• 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
Parts of the Eye
(c) McGraw Hill Ryerson 2007
Human Vision
• 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.
See pages 202 - 203
Parts of the Eyea
b
c
d
Locate the cornea, optic nerve, lens, and retina.
(c) McGraw Hill Ryerson 2007
(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.
See pages 204 - 205
The Cornea-Lens-Retina System
• 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.
(c) McGraw Hill Ryerson 2007
Try the activity (Figure 6.5) on page 205 of your text book!
(c) McGraw Hill Ryerson 2007
Black-and-White Visionand 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.
See page 206
cone
rod
Electron micrograph of the
retina.
(c) McGraw Hill Ryerson 2007
Correcting Focus Problems
See pages 208 - 209
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.
(c) McGraw Hill Ryerson 2007
Correcting Focus Problems
See pages 208 - 209
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.
(c) McGraw Hill Ryerson 2007
Correcting Focus Problems
See pages 208 - 209
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.
(c) McGraw Hill Ryerson 2007
Blindness
See page 209
• 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
(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.
See page 210T
A test for red-green colour vision deficiency.
http://micro.magnet.fsu.edu/primer/java/humanvision/colorblindness/
Take the Section 6.1 Quiz
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