9.5 Light and VisionPages 441-444

The HypothesesDemocritus• Matter composed of atoms

which are carried to the soul to be viewed

Empedocles • Matter composed of 4

elements• Earth, Air, Wind and Fire

and vision must be related to fire

• Light radiated from eyeGalen• Optic nerve conducted

visual spirits

Today we accept the following theories:

Sir Isaac Newton and Christian Huygens

•Particles of light (photons) travel in waves of various lengths

•Light enters the eye as it is reflected or transmitted from objects

Afterimages

Two types of afterimages:Positive Afterimage•Occurs after you look into a bright light

and then close your eyes•The image of the light can still be seen

even with eyes closedNegative Afterimage•Eye is exposed to bright coloured light for

an extended period of time

Focusing the ImageRefraction: slowing of light by dense

material (cornea)

•Light is first bent by the cornea toward the pupil

•It is further bent by the lens to a focal point behind the lens

•Image is projected on the retina

Focusing the Image

Light bent by cornea toward pupil

Light bent by cornea toward pupil

Bent by lens to focal point

Bent by lens to focal point

Image projected on

retina

Image projected on

retina

Accommodation

•The adjustment of the lens to objects far and near

•Objects up close pupil constricts in attempt to bring the image into focus

•Objects far away pupil dilates to in an attempt to capture light

Vision Defects

Glaucoma•Build-up of

aqueous humour•Ducts are blocked • Increases fluid

pressure•Lack of oxygen

and neurons can result in blindness

Cataract•Lens becomes

opaque•Prevents light

from passing through

•Solution: remove lens and use strong eyeglasses or replace with plastic lens

Myopia (nearsightedness)•Eye ball is too long•Distant image is brought into focus in

front of the retina

Hyperopia (farsightedness)•Eye ball is too short•Distant image is brought into focus

behind retina