6.1 Human Vision

1
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
2
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
3
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
4
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.

rod
cone
Electron micrograph of the retina.
See page 206
5
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
6
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
7
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
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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
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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