Updated: May 15, 2022
"It is therefore most appropriate that the eye’s sensation of the light that is in the visible object should occur only through the light passing from the object to the eye"
Ibn Al-Haytham, Book of Optics
Photography is one of my pastimes. I have a digital single-lens reflex camera (DSLR). An SLR camera uses a combination of lens and mirrors so that when you look through the lens, you see exactly what you are photographing. The term camera is from the Latin meaning room or chamber. Before the advent of modern photography, there was the camera obscura which means "dark room".
In its earliest form, it was literally a small, dark room where light entered through a small hole in a wall and projected an inverted image on the opposite wall. Smaller versions were used, beginning in the Renaissance, by artists to aid in drawing and learn perspective. Eventually, portable boxes fitted with a lens instead of a pinhole were created and were the precursors of modern-day cameras. Known familiarly as pinhole cameras, one can easily make a camera obscura from a shoebox.
The camera obscura is based on the concept that light travels in straight lines, called the rectilinear propagation of light. Objects either absorb or reflect different wavelengths of light to determine their color and brightness. The individual rays of light will travel in a straight line from the object. A small opening in a screen will allow only those rays of light coming directly from the viewed object. These rays are then collected on the opposite screen to form an inverted image. The size of the hole greatly affects the sharpness of the image. The smaller the hole, the sharper the image. But there is a trade-off. The image will appear dimmer due to less light. Too small an opening and the image becomes distorted due to the diffraction of light. Mirrors can be used to reverse the image.
At the beginning of the 11th century, an Arab scientist, Ibn Al-Haytham (known to the West as Alhazen), utilized a camera obscura to investigate the nature of light. Al-Haytham is considered one of the first men to follow a purely experimental method when conducting science. His discoveries, as written in his Book of Optics, predated Isaac Newton's own work in optics by nearly six hundred years.
Al-Haytham was born about 935 C.E. in Basra, Iraq. He became known for his use of applied mathematics and claimed he found a method of regulating the flooding of the Nile. This attracted the attention of the Fatimid Caliph Al-Hakim who ruled over much of Northern Africa. Al-Hakim was known to be an eccentric, temperamental, and often brutal ruler. He issued arbitrary edicts and laws as well as executed government officials on mere whims. When Al-Haytham realized that his solution, damning of the Nile south of Aswan, would be impractical, he feared for his life. Legend has it that he feigned insanity to avoid the wrath of the Caliph. He was placed under house arrest for ten years until Al-Hakim's death in 1021. Afterward, he lived in a domed house in Cairo teaching astronomy, physics, and mathematics. During his imprisonment, he wrote a number of works on a variety of topics including physics, astronomy, math, anatomy, medicine, and engineering. It is estimated he wrote more than 200 books of which only 96 are known and of those, only 50 have survived until the modern age. Among these was the Kitab Al Manazer, or the Book of Optics.
Al-Haytham pioneered the use of an experimental method to test hypotheses. Similar to today's scientific method, his method consisted of a cycle of observation, hypothesis, experimentation, and independent verification. He utilized a room-sized camera obscura to perform experiments into the nature of light. He demonstrated how lenses work. In particular, he showed how convex lenses can be used to magnify an image. He investigated the passage of light through various media and discovered the laws of refraction. He performed experiments with prisms to study the dispersion of light into colors long before Newton. He wrote extensively about phenomena like shadows, rainbows, and eclipses. He pondered the physical nature of light and explained how the apparent size of the sun and moon changes near the horizon.
In one experiment, he hung lanterns at varying heights outside the room. Al-Haythem observed that the lanterns illuminated different spots in the darkened room. He was able to trace a direct line from an individual spot of light, through the small hole in the wall, to a specific lantern. By doing this, he proved the underlying principles of the camera obscura and how pinhole cameras function.
In addition, he would cover up different lanterns and discovered their spots of light would darken. The spots reappeared when the lanterns were exposed again. From this, he deduced that light originates from objects.
Most significantly though, through such experimentation and observation such as this, Al-Haytham developed a better understanding of human vision and how the human eye works.
At the time, there were two competing theories. The first was the emission theory. In this theory, it was thought that light rays originated within the eye and were emitted to produce sight. This theory had its roots with the Ancient Greek thinkers Euclid and Ptolemy. The other theory was the intromission theory. This took the opposite view in that vision originated from outside the eye. Aristotle was an early supporter of this theory, and he described vision as the form of an object traveling from the object to the eye. Through his experimentation, Al-Haytham garnered evidence for the intromission theory. He concluded that light reflects from objects and travels in straight lines to the eye. But he went a step further. He attempted to describe the function of the eye at the anatomical level. In this, he wedded the geometry of Euclid to Aristotle's forms.
Al-Haytham deduced that the eye operates just like a camera obscura. Light travels in straight lines from an object to the lens of the eye which acts like the pinhole. The inverted image would strike the back of the eye. He postulated that the light entering the eye is additionally refracted in order to make the image right-side-up before hitting the optic nerve. He synthesized anatomy with the physics of light and optics. He went on to add to our understanding of binocular perception and visual acuity.
Al-Haytham's work, translated into Latin, was introduced to the West during the 13th century. He would continue to exert an influence on Western scientists up to the modern era.
This knowledge of the properties of light and the action of lenses led to practical applications during the 13th century. First spectacles. Then telescopes, microscopes, and eventually cameras. The invention of photographic paper allowed the leap from the old camera obscura, an aide for drawing, to a device that can permanently record an image. Early cameras were simple pinhole cameras that would evolve into our modern-day digital SLR cameras. Yet, perhaps the greatest camera resides in our head. Our eyes.
To learn more about how Arab scientists influenced Western thought: