The Modernity of Ibn al Haytham (965-1039)- Part2

Ibn al Haytham (Alhazen) is considered the initiator of modern optics and the scientific method.

     the Eye :

By multiple experiments, with camera obscura (darkened chamber) in particular, Ibn al-Haytham showed that:

"Sight does not perceived any visible object unless there exists in the object some light which the object possesses of itself or which radiates upon it from another object"

• light rays emanate in all directions from each point on surface of all llaminated objects
  
• light rays intersect with each other without interference.

Ibn al-Haytham proves the theory of intromission according to which light enters the eye. It proves that all objects reflect light in all directions, but it is when a ray collides with the eye that we see theobject reflecting the ray. Thus the eye can perceive the shape, the color, the transparency as well as the movement of the object. He also proved that indeed each of the two eyes captures an image even if only one is seen. He contradicted Ptolemy on the fact that the eye would emit light.
According to him, if the eye was designed that way we could see at night. He understood that sunlight was scattered by objects and then entered the eye.

"Straight lines [exist between] the surface of the eye [and] every point on the surface of an object.

An experimental study of this fact can easily be carried out with the help of rulers and tubes.

 If a part of the opening is closed, only that part of the object which is on a straight line between the eye and the body observed will be masked, the straightness being verified by the stralghtness of the ruler and the tube. It results from this experiment, with an evidence that dispels doubt, that the eye does not perceive as visible any object located in the same siruation, this perception can only be realized by the reflection of light and by following straight lines that can be extended by imagination between the surface of the object and the surface of the eye. The view does not perceive any object unless there is some light from the object, either the object itself is bright or it is illuminated by the radiant light of another object."

"Illuminated objects emit light in all directions from the light impact on their surface. When the eye is facing an illuminated object, it is located on the light path that starts from the object. And as the property of light is to affect the sight and as the characteristic of the eye is to be sensitive to light then the sight is exerted thanks to the light which leaves the object and reaches the eye."



Ibn al-Haytham indicated vision occurs in the brain not in the eyes. Eyes are detectors, brain is an interpreter of reality. Scientific method is necessary to get a faithful picture of reality and not an illusory picture.

                     The scientific method

Ibn al-Haytham introduces his method of research into the introdction to his Book of Optics

as follows :
"We should distinguish the properties of particulars, and gather by induction what pertains to the eye and what is found in the manner of sensation to be uniform, unchanging, manifest and not subject to doubt. After which we should ascend in our enquiry and reasoning, gradually and orderly criticizing premises and exercising caution in regard to conclusions our aim in all that we make is subject to inspection and review, not to follow prejudice, and to take care in all that we judge and

criticize that we seek the truth and not be swayed by opinion"

He formulated a theory about judgment and recognition of objects. He notices that we only recognize objects we know, and that the image of an object persists some time after we close our eyes. Recognition is based on memory and is not just a feeling, becasse we do not immediarely recognize objects that are unknown to us.
Ibn al-Haytham, in developing his theory of optics, founded his realistic, materialistic, objective vision of the world. It practically distinguishes theeye is thus the prototype of the detector whose existence is inlependent of the observed object.

Ibn al-Haytham is the first to use scientific method sysematically :
(1) Start from existing observations and theoretical elements with a critical attitude

(2) Define the project and formulate a hypothesis

(3) Testing the hypothesis through controlled experimentation and analysis of results

(4) Interpret them and draw conclusions (confirming, improving or changing the existing theory)

(5) Publication of results

(6) Publication of description of different steps of measurements and of measurement instruments

in order to allow verification of results by others.

      Methodical doubt<br>

Ibn al-Haytham practices methodical doubt by subjecting not only the writings of the ancients but also his own prejudices to ruthless criticism. Three examples

1. "The seeker after truth is not one who studies the writings of the ancients and, following his natural disposition, puts his trust in them, but rather the one who suspects his faith in them and
   
   questions what he gathers from them, the one who submits to argument and demonstration and not the sayings of human beings whose nature is fraught with all kinds of imperfection and deficiency.

Thus 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, and, applying his mind to the core and margins of its content, attack it from every side. he should also suspect himself as he performs his critical examination of it, so that he may avoid falling into either prejudice or leniency".

2"The search for truth is arduous, the road that leads to it is full of pitfalls. To find the truth, one must set aside one's opinions and not trust the writings of the elders. You must question them and submit each of their assertions to your critical spirit. Rely only on logic and experimentation, never on the affirmation of one another, for every human being is subject to all sorts of imperfections; in our quest for truth, we must also question our own theories, with regard to each of our research to avoid succumbing to prejudice and intellectual laziness. Do so and the thruth will be revealed to you."

3- Here is how Ibn al-Haytham responds in a scathing way to an anonymous contradictor who opposes his scientific conceptions: " From the statements made by the noble Sheikh, it is clear that he believes in Ptolemy's words in everything he says, without relying on a demonstration or calling on a proof, but by pure imtation; that is how experts in the prophetic tradition have faith in

Prophets, may the blessing of God be upon them. But it is not the way that mathematicians have faith in specialists in the demonstrative sciences. And I have taken note that it gives him [i.e. the

Sheikh] pain that I have contradicted Ptolemy, and that he finds it distasteful; his statements suggest is foreign to Ptolemy. Now there are many errors in Ptolemy, in many passages of his

that error books. If he wishes me to specify them and point them out, I shall do so."

Thank you for reading.

The picture
The Modernity of Ibn al Haytham (965-1039)- Part