Human Vision

Correcting Focus ProblemsNear-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.

Correcting Focus ProblemsFar-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.

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.

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.

cone

rod

Electron micrograph of the

retina.

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

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.