(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

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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.

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