READ: The Universe Through a Pinhole: Hasan Ibn al-Haytham

By Bennett Sherry

Isaac Newton said he saw further because he stood on the shoulders of giants, but historians have often ignored just how true this was. Newton’s understanding of his actual ability to see, and to comprehend the things he saw, was thanks in part to one invisible giant: Hasan Ibn al-Haytham.

Our understanding of the Universe—of the stars and celestial bodies that travel across the night sky—depends on our ability to see light. Once scientists understood how light moves and how it reaches us, so many new discoveries were possible. For example, one of the most important advances in the study of physics was the invention of the telescope. But the telescope was only made possible by an understanding of the science behind light and how we see it, a science called optics.

Ibn al-Haytham’s most important book was Kitab al-Manazir, which is Arabic for The Book of Optics. This book explained how the human eye works and how we see objects, such as stars, that are very far away. After Ibn al-Haytham’s book was translated from Arabic into Latin around 1200 CE, it sparked a revolution in optics in Europe. His knowledge provided the basis for many of the great scientific discoveries of later scholars such as Galileo and Kepler.

That’s impressive, but what makes Ibn al-Haytham special was how he approached science. He also stood on the shoulders of giants, but he didn’t just stand there. He learned from the knowledge of Greek scholars like Euclid, Aristotle, and Ptolemy, but he also challenged their ideas. To challenge the ancient ideas of these giants, Ibn al-Haytham used the scientific method—and this was 500 years before the Scientific Revolution. The scientific method is the process of asking a question, developing a hypothesis, and testing that hypothesis through rigorous experiments. By using this method, long before it was widely accepted, al-Haytham became one of the giants himself.

Ibn al-Haytham was born in 965 CE in the city of Basra (in present-day Iraq). This part of the world was a center of science and learning at the time. In Basra, Ibn al-Haytham became famous for his mathematical ability. But the young man was perhaps too confident. He claimed that he could build a dam to control the flooding of the great Nile River. When the ruler of Egypt heard of Ibn al-Haytham’s claim, he was very interested in bringing it to life, and invited Ibn al-Haytham to Cairo. Yet, once Ibn al-Haytham stood on the banks of the Nile, he realized his mistake. There was no way he could build a dam large enough. The ruler of Egypt was angered, and Ibn al-Haytham spent the next years either in hiding or imprisoned in his house.

But all was not lost. Ibn al-Haytham’s time in isolation helped him see the world in a different light—literally. One night, sitting in a dark room, he noticed moonlight passing through a tiny hole in the wall. Where the light hit the wall on the other side of the room, an image of the Moon was projected. But it was upside down! Why? This question led Ibn al-Haytham to launch a series of experiments to verify a theory he had formed, based on his observation. He reproduced the effect he had seen in the moonlight by building a camera obscura (see Figure 1) and documenting his observations. These observations seemed to have certain patterns, and Ibn al-Haytham developed mathematical explanations for these patterns.

Some ancient philosophers believed that human eyes could shoot rays of light outward in a cone, and that when these rays hit objects, we could see them. Ibn al-Haytham was suspicious of this idea and challenged it. This challenge was a big deal—people respected ancient ideas, and many assumed them to be true. Although Ibn al-Haytham might have respected these ancient thinkers too, he wasn’t willing to accept their theories without testing them first. He took the theories of scholars like Euclid, Ptolemy, Galen, and Aristotle and combined them with newer ideas from Arab thinkers. By challenging and improving on these ideas, he developed new theories of light and sight. He backed up his theories with repeated testing and verification, an aspect of the scientific method that scientists still practice today.

Ibn al-Haytham’s big claim was that humans see things because rays of light reflect off of objects in straight lines that then travel to our eyes. That’s probably something you’ve already learned, but without the work of Ibn al-Haytham, you might have been taught that we’re able to see because our eyes shoot beams of light! Fun to think about, but 100 percent mistaken! At the time, and for centuries to come, Ibn al-Haytham’s ideas were revolutionary. His observations in that dark room also allowed him to mathematically prove that the Moon appears bright because of sunlight reflecting off of it. These observations led him to understand that our eyes are connected to our brain by optical nerves. His work on optics allowed later scientists to stand on his shoulders as they developed innovations like telescopes, microscopes, cameras, and eyeglasses. His dozens of books and his experiments helped other scientists better understand how we perceive objects in the night sky. There’s even a crater on the Moon named after him!

Some historians consider Ibn al-Haytham the first scientist because of his rigorous processes for proposing and then testing theories. He used this method in his own work and encouraged others to do so in his books. He wrote:

In this quote, Ibn al-Haytham is urging other scientists to use the scientific method. In other words, we shouldn’t trust old ideas. We should make ourselves “an enemy” of everything we read: always questioning, always testing. Thanks to the work done by Ibn al-Haytham in the tenth and eleventh centuries, later scholars in the Islamic world and Europe—including Isaac Newton—were able to develop new theories about gravity and the movement of stars and planets.

Camera obscura is a Latin phrase meaning “dark chamber.” When the light of the Moon passed through a small hole into Ibn al-Haytham’s dark room, it produced an effect that humans have known about and been experimenting with since at least the fourth century in ancient Greece and China. Ibn al-Haytham wanted to know why the projection produced an upside-down image. This effect helped him understand that light moves in straight lines. When light is reflected off an object, it travels in straight lines away from the object in all directions. So, for example, when that light reflecting from the top of an object hits a tiny opening, like the hole in Ibn al-Haytham’s room, it must travel in a straight line. So, it can only land at the bottom of the room. Likewise, light reflecting off the bottom of the object can only land at the top of the room. That’s why the Moon appeared upside down on his wall.

Among his many contributions to human knowledge, Ibn al-Haytham changed the way we “see” how eyesight actually works. By observing the behavior of light in the camera obscura, he was able to understand that something similar must happen in our eyes. And indeed, he was correct! As light passes through the tiny opening in our eye—the pupil—it creates an upside-down projection on the back of our eye. This revelation led Ibn al-Haytham to another: he was the first to argue that vision happens in the brain, not the eyes. Our brain reinterprets the upside-down projection of light, so we see things right-side-up. His book included detailed diagrams, including the one you see him drawing here, which labeled the different parts of the eye and showed the pathways of the optical nerves.

Bennett Sherry holds a PhD in history from the University of Pittsburgh and has undergraduate teaching experience in world history, human rights, and the Middle East. Bennett writes about refugees and international organizations in the twentieth century. He is one of the historians working on the OER Project courses.