The Science Book

THE BEGINNING OF SCIENCE 29

Greek scholars Euclid and, later,
Ptolemy believed that vision
derived from “rays” that beamed
out of the eye and bounced back
from whatever a person was looking
at. Alhazen showed, through
the observation of shadows and
reflection, that light bounces off
objects and travels in straight lines
into our eyes. Vision was a passive,
rather than an active, phenomenon,
at least until it reached the retina.

He noted that, “from each point of

every colored body, illuminated

by any light, issue light and color

along every straight line that

can be drawn from that point.”

In order to see things, we have only

to open our eyes to let in the light.

There is no need for the eye to send

out rays, even if it could.

Alhazen also found, through his

experiments with bulls’ eyes, that

light enters a small hole (the pupil)

The duty of the man

who investigates the

writings of scientists, if

learning the truth is his

goal, is to make himself an

enemy of all that he reads.

Alhazen

Alhazen

Abu Ali al-Hassan ibn al-

Haytham (known in the West as

Alhazen) was born in Basra, in

present-day Iraq, and educated

in Baghdad. As a young man he

was given a government job in

Basra, but soon became bored.

One story has it that, on hearing

about the problems resulting

from the annual flooding of

the Nile in Egypt, he wrote to

Caliph al-Hakim offering to build

a dam to regulate the deluge,

and was received with honor

in Cairo. However, when he

traveled south of the city, and

saw the sheer size of the river—

which is almost 1 mile (1.6 km)

wide at Aswan—he realized the

task was impossible with the

technology then available. To

avoid the caliph’s retribution he

feigned insanity and remained

under house arrest for 12 years.

In that time he did his most

important work.

Key works

1011–21 Book of Optics

c.1030 A Discourse on Light

c.1030 On the Light of the Moon

See also: Johannes Kepler 40–41 ■ Francis Bacon 45 ■ Christiaan Huygens 50–51 ■ Isaac Newton 62–69

and is focused by a lens onto a

sensitive surface (the retina) at

the back of the eye. However, even

though he recognized the eye as a

lens, he did not explain how the

eye or the brain forms an image.

Experiments with light

Alhazen’s monumental, seven-

volume Book of Optics set out his

theory of light and his theory of

vision. It remained the main

authority on the subject until

Newton’s Principia was published

650 years later. The book explores

the interaction of light with lenses,

and describes the phenomenon of

refraction (change in the direction)

of light—700 years before Dutch

scientist Willebrord van Roijen

Snell’s law of refraction. It also

examines the refraction of light

by the atmosphere, and describes

shadows, rainbows, and eclipses.

Optics greatly influenced later

Western scientists, including

Francis Bacon, one of the scientists

responsible for reviving Alhazen’s

scientific method during the

Renaissance in Europe. ■

Alhazen provided the first scientific

description of a camera obscura, an

optical device that projects an

upside-down image on a screen.

Object

Light rays

travel from

the object

Pinhole

Image is upside down

and back to front