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How do you see this sentence? Many ancient thinkers believed the eye sends out rays that touch what it looks at. Ibn al-Haytham, writing about a thousand years ago, argued from observation and experiment that it works the other way round: light comes from objects into the eye. This post covers who he was, what his Book of Optics contains, and a pinhole-camera problem that uses the same straight-line geometry he relied on.
Who was Ibn al-Haytham?
Ibn al-Haytham was probably born around 965 in Basra, in present-day Iraq, and probably died around 1040 in Cairo, according to the MacTutor biography, which marks both dates as uncertain. In Latin Europe he was known as Alhazen.
MacTutor records an account from the historian al-Qifti: the Fatimid ruler al-Hakim invited him to Egypt to regulate the Nile's flow, and when Ibn al-Haytham realised the plan would not work, he feared the ruler and pretended to be mad until al-Hakim died in 1021. MacTutor notes that the sources disagree on details, so treat this as a story told about him rather than settled fact. He is credited with around 92 works, more than 55 of which survive.
What is in the Book of Optics?
His seven-volume Kitab al-Manazir is usually seen as his most important work. MacTutor summarises its parts:
Book I: light and vision. He argued that light is the same in nature whatever its source, whether sunlight, firelight or light from a mirror, and gave the correct explanation of vision: light reflected from an object enters the eye.
Books II and III: how we perceive what we see, and what causes errors in vision.
Book IV: reflection, with experimental proof and a full statement of the laws of reflection.
Book V: what became known as Alhazen's problem: given a light source and a spherical mirror, find the point where light is reflected to an observer's eye.
Books VI and VII: errors in vision caused by reflection, and refraction, which he explained by supposing light moves more slowly in denser media.
Book I also insists that the study will be based on experimental evidence rather than abstract theory. That stance is why he is often discussed in the history of the scientific method.
Why does the year 2015 matter?
The United Nations General Assembly resolution 68/221 proclaimed 2015 the International Year of Light and Light-based Technologies. It noted that 2015 marked several anniversaries, starting with "the works on optics by Ibn Al-Haytham in 1015", alongside Fresnel's wave idea of 1815 and Maxwell's electromagnetic theory of 1865. For Fresnel's part of the story, see our post on Augustin Fresnel and the wave theory of light.
Worked example: a pinhole camera
Ibn al-Haytham studied images formed through small openings. A pinhole camera works because light travels in straight lines, the same assumption behind every ray diagram you draw.
A tree 6 m tall stands 20 m from a pinhole camera. The screen is 25 cm behind the hole. How tall is the image?
Rays from the top and bottom of the tree cross at the pinhole, forming two similar triangles.
Image height ÷ object height = image distance ÷ object distance.
Image height = 6 m × (0.25 m ÷ 20 m) = 6 × 0.0125 = 0.075 m = 7.5 cm.
The image is inverted, because the rays cross at the hole.
Follow-up. If you move the camera to 40 m from the tree, the image halves to 3.75 cm. If you double the screen distance to 50 cm, it doubles to 15 cm.
What can you check at home?
Make a pinhole camera from a shoebox with tracing paper as the screen. Measure a window's height and distance, predict the image size with the ratio above, and then measure it. If your prediction and measurement differ, list what could cause it: a hole that is too large, a screen that is not flat, or distances measured from the wrong point. This is the habit Ibn al-Haytham argued for: test the claim and examine your errors.
What can a student learn from Ibn al-Haytham?
Doubt respectfully. He wrote a whole work, Doubts concerning Ptolemy, questioning the most respected astronomer of his tradition, as MacTutor describes.
Back claims with experiments. His case for how vision works rested on what you can observe and test.
Turn physics into geometry. Straight-line light and similar triangles let him predict, not just describe.
Know the limits of your model. MacTutor notes his account of the eye went wrong because he lacked the idea of the eye's lens forming an image.
Ray diagrams make more sense when you can move the object and watch the image change. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For the board-exam version of these ideas, read our guide to light, reflection and refraction in Class 10.
This weekend, build the shoebox camera and check whether your measured image matches your predicted 1:80 ratio for a 25 cm box at 20 m.
