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Abu Ali
Hasan Ibn al-Haitham, or Alhazen, as he is known in the
West, was one of the most eminent Moslem physicists,
whose contributions to optics and the scientific methods
are outstanding. He was born in 965 A.D.in Basra, Iraq
and received his education in Basra and Baghdad. Ibn al-Haitham,
who died in 1039 AD in Cairo, was a great scientist and
engineer. On a trip to Egypt to study the Nile River, he
recommended building a dam at Aswan to store the summer
floodwater upstream and use it throughout the year. This
project was finally executed in the 20th century!
He also travelled to Spain
and, during this period, he had ample time for his
scientific pursuits, which included optics, mathematics,
physics, medicine and development of scientific methods
on each of which he has left several outstanding books.
His extensive researches on optics, has earned him the
title “ Father of modern Optics”.
Ibn al-Haitham made a thorough
examination of the passage of light through various
media and discovered the laws of refraction. He also
carried out the first experiments on the dispersion of
light into its constituent colours. His book
Kitab-al-Manadhir was translated into Latin in the
Middle Ages and taught in European colleges, along with
his book that was dealing with the colours of the
sunset. He described in details the physical phenomena
of shadows, eclipses, and rainbow and speculated on the
physical nature of light. Al Hazen was the first scholar
to describe accurately the various parts of the eye and
give a scientific explanation of the process of vision.
He also attempted to explain binocular vision, and gave
a correct explanation of the apparent increase in size
of the sun and the moon when near the horizon. He
contradicted Ptolemy's and Euclid's theory of vision
that objects are seen by rays of light emanating from
the eyes; according to him the rays originate in the
object of vision and not in the eye.
We know little of Ibn al-Haytham's
years in Basra. It appears that he did not devote
himself to the study of mathematics and other academic
topics at a young age but trained for what might be best
described as a civil service job. He was appointed as a
minister for Basra and the surrounding region. However,
Ibn al-Haytham became increasingly unhappy with his deep
studies of religion and made a decision to devote
himself entirely to a study of science, which he found
most clearly described in the writings of Aristotle.
Having made this decision, Ibn al-Haytham kept to it for
the rest of his life devoting all his energies to
mathematics, physics, and other sciences.
Ibn al-Haytham went to Egypt
some considerable time after he made the decision to
give up his job as a minister and to devote himself to
science, for he had made his reputation as a famous
scientist while still in Basra.
In
Cairo, where he lived part of his life, he spent much of
his time conducting experiments, of which many involved
a dark room with a hole in it. He hung five lanterns
outside the room, adjacent to the wall with the hole,
and noticed that there were five 'lights' on the wall
inside his dark room. He would then place an obstruction
between one of the lanterns and the hole, and observed
how one of the 'lights' on the wall disappeared.
Furthermore the lantern, the obstruction and the hole
were in a straight line.
This demonstrated that light
travelled in straight lines and that, even though the
light from the five lanterns all travelled through the
little hole at the same time, it did not get mixed up:
there were five 'lights' on the wall inside the room.
Ibn al-Haitham deduced that this is how the eye worked,
which had been the subject of a long debate. Aristotle
had believed that the eye sent out rays to scan objects,
but Al Hazen believed that the opposite is true - that
light was reflected into the eye from the things one
observed, thus overturning a thousand years of
scientific thought. His experiment was the first
scientific description of the 'camera obscura' (dark
room), the principle behind the pinhole camera.
In
detailing his experiment with spherical segments (glass
vessels filled with water), he came very close to
discovering the theory of magnifying lenses, which was
developed in Italy three centuries later. It took
another three centuries before Snell and Descartes
proposed the law of sines.
Ibn al-Haitham's scientific
approach differed from that of the Ancient Greeks in
that they saw that truth was determined by the logic and
beauty of reasoning, and when experiment was used it was
only as a demonstration. That's the reason why Ptolemy,
even though he did experiments, has supported the
erroneous "emission" theory of vision. In contrast, Ibn
al-Haitham saw experiments as being the essential factor
that distinguished a true theory from a false one -
through this insight he created the foundation for the
"scientific method".
Al
Hazen’s research in catoptrics focused on spherical and
parabolic mirrors and spherical aberration. He made the
important observation
that the ratio between the angle of incidence and
refraction does not remain constant and investigated the
magnifying power of a lens. His catoptrics contains the
important problem known as “ Alhazen's problem”. It
comprises drawing lines from two points in the plane of
a circle meeting at a point on the circumference and
making equal angles with the normal at that point. This
leads to an equation of the fourth degree. He also
solved the shape of an aplantic surface for reflection.
Ibn al-Haytham's writings are too
extensive for us to be able to cover even a reasonable
amount. It seems that he has written over 200 scientific
works, of which remarkably, 55 have survived. The main
topics on which he wrote were optics, including a theory
of light and a theory of vision, astronomy and
mathematics, including geometry and
number theory. We will
give at least an indication of his contributions to
these areas.
In his book Mizan al-Hikmah,
Ibn al-Haitham has discussed the density of the
atmosphere and developed a relation between it and the
height. He also studied atmospheric refraction and
discovered that the twilight only ceases or begins when
the sun is 19° below the horizon and attempted to
measure the height of the atmosphere on that basis. He
has also discussed the theories of attraction between
masses, and it seems that he was aware of the magnitude
of acceleration due to gravity.
Ibn al-Haitham's contribution
to mathematics and physics is extensive. In mathematics,
he developed analytical geometry by establishing linkage
between algebra and geometry. In physics he studied the
mechanics of motion of a body and was the first to
propose that a body moves perpetually, unless an
external force stops it or changes its direction of
motion. This is strikingly similar to the first law of
motion. He has also discussed the theories of attraction
between masses.
A seven-volume work on optics,
Kitab al-Manazir, is considered by many to be ibn
al- Haitham's most important scientific contribution. It
was translated into Latin as Opticae thesaurus
Alhazeni in 1270. The previous major work on optics
had been Ptolemy's Almagest and although ibn al-
Haitham's work did not have an influence to equal that
of Ptolemy's, it must be regarded as the next major
contribution to the field. The work begins with an
introduction, in which ibn al- Haitham says that he
will begin "the inquiry into the principles and
premises". His methods will involve "criticising
premises and exercising caution in drawing conclusions"
while he aimed "to employ justice, not follow prejudice,
and to take care in all that we judge and criticise that
we seek the truth and not be swayed by opinions".
Also in Book I, Ibn al-
Haitham makes it clear that his investigation of light
will be based on experimental evidence rather than on
abstract theory. He notes that light is the same
irrespective of the source and gives the examples of
sunlight, light from a fire, or light reflected from a
mirror, which are all of the same nature. He gives the
first correct explanation of vision, showing that light
is reflected from an object into the eye. Most of the
rest of Book I is devoted to the structure of the
eye but here his explanations are in error since he does
not have the concept of a lens, which is necessary to
understand the way how the eye functions. His studies of
optics did lead him, however, to propose the use of a
camera obscura, and he was the first person to mention
it.
Book II of the
Optics discusses visual perception while Book III
examines conditions necessary for good vision and how
errors in vision are caused. From a mathematical point
of view Book IV is one of the most important
since it discusses the theory of reflection.
Book VI of the
Optics examines errors in vision due to reflection
while the final book, Book VII, examines
refraction.
Abu al-Qasim ibn Madan was an
astronomer who proposed questions to ibn al-Haytham,
raising doubts about some of Ptolemy's explanations of
physical phenomena. Ibn al- Haitham wrote a treatise
Solution of doubts in which he gives his answers to
these questions.
Ibn al- Haitham’s main purpose
in Analysis and synthesis is to study the methods
mathematicians use to solve problems. The ancient Greeks
used analysis to solve geometric problems but ibn al-
Haitham sees it as a more general mathematical method,
which can be applied to other problems such as those in
algebra. In this work ibn al- Haitham realises that the
analysis was not an algorithm thath could automatically
be applied using given rules but he realises that the
method requires intuition.
Ibn al-Haitham's influence on physical sciences in
general, and optics in particular, has been held in high
esteem and, in fact, it ushered in a new era in optical
research, both in theory and practice. | 
