Until Galileo Gelilei the assumption was that all objects tend resist movement to stay at rest. Until Antoine Lavoisier the assumption was the burning an object will cause the destruction of matter and disappearance of mass. Until Louis Pasteur the assumption was that living organisms would commonly descend from non-living objects--flies plainly come from putrid meat, aphids from dew, mice come from haystacks, fish come from water, alligators come from wet rotting logs, etc. Until Hermann Joseph Muller the assumption was that radioactivity was healthy and invigorating for the human body. (All fine stories those that we will get to in due time.) The point is that “common knowledge” certainly does not always reflect reality, and oftentimes experimentation results in unexpected, even counter-intuitive, sometimes baffling results.
Experimentation is all about constructing a test to measure something and help decide between competing hypotheses; this is a more conclusive method than the old guessing of intuitive reasoning.
The first noted experimentalist was the 11th century CE Iraqi physicist Ibn al-Haytham (بن الهيثم, also sometimes simply “Alhazen”), someone who actively rejected Aristotelian intuitive reasoning and attempted to prove a hypothesis using observation from constructed experiments. Al-Haytham was born in 965 CE in the city of Basra of modern Iraq, in the Persian Empire. Records depict Al-Haytham as a brilliant, proud, confident, but cowardly man. Legend has that he boldly proclaimed that he could study and control the periodic flooding of Nile River. When the ruling Caliph demanded this task from the boastful Al-Haytham, the physicist quickly realized he could not do it, so he faked insanity to save himself from the wrath of the Caliph. He was then placed under house-arrest, where he quietly conducted most of his experiments.
Al-Haytham is most famous for his study of light. He attempted to address a recurring ancient debate about optics and vision: Do objects emit something that reaches the eyes, or do the eyes emit some beam that reaches and detects objects? This question might seem preposterous to us in the 21st century, but by far the most commonly accepted theory at that time was that the eyes send out a beam like a sort of radar that allows us to perceive the world. As cool as that would be to have radar-eyes, Al-Haytham developed his hypothesis that the eyes receive rather than send a signal, reasoning, among other things, that stars should be too far away for a eyes to send a signal to, and reasoning that the existence of darkness should preclude the emission hypothesis. After all, if the eyes can send out a signal, why would they stop doing it at night?
He used several experiments to show that beams of light travel in straight lines from a source. In one experiment, he positioned a straight tube and a curved tube next to a light source. Light could pass through the straight tube, but not the curved one, therefore the light must only move in straight lines.
Another experiment was a famous pinhole projection model. The idea was that in a dusty, darkened room, one could see the straight path of light pass into the room. Al-Haytham expanded upon this simple idea to develop a device called a camera obscura (Latin for “darkened room”). He predicted, and then showed, that he could produce a crude projected image of an object by forcing light through a tiny pinhole. Much like the projector at a movie theatre, he could see the dust in the path of the projecting light. He also noted that narrowing the pinhole produced a sharper, but dimmer, image. Al-Haytham’s conclusive experiment on the cause of vision involved him viewing pinhole projections of oil-lamps switching on and off.
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| As you can see, the requirement for light to "squeeze" at a precise angle through the narrow pinhole results in the projected image appearing inverted. |
Later developments were remarkably advanced; Al-Haytham’s treatise Book of Optics, completed some time around 1021 CE, is one of the most influential in the history of physics, including numerous experiments on refraction, reflection, lenses, and mirrors, and descriptions of the atmospheric refraction that causes phenomena such as rainbows and twilight (resulting in a rather accurate calculation of the height of Earth’s atmosphere). Reflection, al-Haytham observed, is the change of direction of a light ray at the surface of a medium interface (where two different media meet). Refraction is the change of direction of a light ray as it passes from one medium to another, like from air into water.
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| In this diagram representing refraction, we can see that the angle changes when the ray changes media: once from air-to-glass, and then again from glass-to-air. The dashed line represents the apparent path of the ray, had it been unimpeded. |
Different colors of light refract differently--red refracts least, and violet refracts most. (We will delve into this when we talk about the physical nature of light.) Al-Haytham was able to accurately explain the phenomenon of twilight, understanding that as the light passes from the sun to our atmosphere the air causes light to refract. When the light from the sun approaches at a certain angle, the red light reaches our eyes while the other colors tend to refract away. Rainbows occur when light at a certain angle passes from air into water droplets and then back to the air.
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| Al-Haytham showed that perceived white light is actually a mixture of colored lights (thus lending way for 'Dark Side of the Moon' about 950 years later). |
He speculated on the physical nature of light that might cause such behavior. He also dissected eyes and attempted to assign explanations to the anatomy, though he incorrectly identified the eye’s lens as the receptive tissue, whereas we know now that it is the retina on the back of the eye that detects light. Al-Haytham’s mathematical solutions to some of his problems were generations ahead of his time--he came very close to inventing methods that what we now call calculus.
As a legacy, Al-Haytham’s most important contribution was that he developed and described abstract theories from inductive reasoning, and then systematically evidenced his predictions. Translations of these works and others did not reach Europe until around the early-13th century, where they may have inspired modern methodologies of later scientists like Roger Bacon, Galileo, and others whose work we will discuss in depth another day.
Important Ideas
- The Iraqi physicist Ibn Al-Haytham was a trailblazer of the scientific method of observation, testing, and measurement of hypotheses in order to reach verifiable conclusions.
- Al-Haytham’s experiments led to numerous significant advances laying the foundation for physical optics.
Other Interesting Reading
- Ibn Sahl, a Persian optics engineer who laid the foundation for a lot of al-Haytham’s work
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