Historical figure · Pearson Edexcel International GCSE revision on Incandio

Ibn al-Haytham

Who Tested What the Ancients Wrote · Scholar of Optics · c. 965–1040

The scholar who settled how vision works by experiment rather than argument, established that light travels in straight lines into the eye, and insisted that the writings of the ancients be tested rather than trusted.

Defend these ideas against Ibn al-Haytham

Incandio is a free revision platform for the Pearson Edexcel International GCSE, and on it a topic counts as mastered only once you have learned it, proved it in questions, taught it back and defended it against the person with the strongest claim on it. Ibn al-Haytham is that person for 4 topics on the course:

• Reflection and Ray Diagrams — Physics
• Refraction and Refractive Index — Physics
• Angles & Parallel Lines — Mathematics
• Circle Properties & Tangents — Mathematics

Who was Ibn al-Haytham?

Ibn al-Haytham was born in Basra around 965 and spent his working life in Cairo under the Fatimid caliphate. The Greeks had left vision unresolved: Euclid and Ptolemy held that the eye sends out rays to feel objects, while others held that objects send something to the eye. He settled it experimentally. Light travels in straight lines from every point of an object in all directions, and vision occurs when some of that light enters the eye — for if the eye emitted rays, why should it hurt to look at the Sun, and why can we not see in the dark? He darkened a room and admitted light through a small aperture, obtaining an inverted image of the scene outside, the earliest clear account of the camera obscura. He studied reflection from plane and curved mirrors, measured the bending of light entering water, and showed that Ptolemy's claim of a simple proportionality between the angles was wrong — though he did not find the correct law. His Kitab al-Manazir, the Book of Optics, was translated into Latin as the work of Alhazen and shaped European optics for six hundred years. He is said to have escaped a caliph's commission to dam the Nile, which he had realised was impossible, by feigning madness, and to have spent years confined as a result.

Major achievements

• Established that light travels in straight lines and enters the eye, refuting emission theories of vision
• Wrote the Kitab al-Manazir, the foundational work of optics for six centuries
• Gave the earliest clear description of the camera obscura
• Studied reflection from plane and curved mirrors systematically
• Showed that Ptolemy's account of refraction was wrong

Life in brief

• c. 965 — Born in Basra: In what is now southern Iraq.
• c. 1010 — Summoned to Cairo: To dam the Nile, a project he realised was impossible.
• c. 1011 — Years of confinement: Said to have feigned madness to escape the caliph's displeasure.
• c. 1021 — The Book of Optics: Vision explained by light entering the eye, established by experiment.
• c. 1021 — The darkened chamber: An inverted image of the world through a small aperture.
• c. 1040 — Dies in Cairo: His work would reach Europe two centuries later.

Explore the life of Ibn al-Haytham — five chapters

1. Basra to Cairo — A scholar summoned to dam the Nile
2. Which Way Does Seeing Go? — Two arguments and one experiment
3. The Darkened Chamber — An inverted world on a wall
4. The Law He Could Not Find — Knowing Ptolemy is wrong is not the same as being right
5. Alhazen — Six centuries of influence under a borrowed name

Read the five-chapter life of Ibn al-Haytham →

Where Ibn al-Haytham appears in your course

Ibn al-Haytham has a genuine claim on 2 lessons of the Pearson Edexcel International GCSE science course built into Incandio:

• Reflection and Ray Diagrams — Physics: Every ray diagram on this page assumes two things that had to be discovered: that light travels in straight lines, and that vision happens when light enters the eye rather than when the eye reaches out. Both are his. Working in Cairo around 1021, he argued that if the eye emitted rays it should not hurt to look at the Sun and we should be able to see in the dark, and he demonstrated rectilinear propagation with a darkened room and a small aperture that produced an inverted image of the world outside. He also studied reflection from plane and curved mirrors in detail.
• Refraction and Refractive Index — Physics: Ptolemy had published tables claiming that the angle of refraction was simply proportional to the angle of incidence, and it had stood for eight centuries. Ibn al-Haytham measured the bending of light entering water and glass carefully enough to establish that it is not — the ratio drifts as the angle grows. He could not supply the correct relationship, and said so; the sine law came six hundred years later. He is the right figure for a page about a law, because he shows what it looks like to be certain that the received rule is wrong while being unable to replace it.

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