This is the newest post in category .PHYSICS
start of synopsis > Ibn al-Haytham
(Alhazen) corrected an error made by Ptolemy concerning the index of refraction. [A Jew who converted to Islam made this correction before Alhazen did.]
Alhazen calculated reflection off a sphere.
Alhazen invented correct explanations of why stars seemed to slow down a few minutes before they set, and why the moon appears to be larger when it is on the horizon.
The camera obscura (pinhole camera) had originally been invented in 5th century BC China, and had been used by Greeks.
Alhazen’s theory of light was identical to that of Euclid.
Alhazen’s incorrect explanation of how the eye worked was copied from Galen of Pergamon.
Alhazen’s incorrect explanation of rainbows was copied from Seneca the Younger.
Alhazen gave incorrect explanations of why sunset is red and why the sky is blue.
Alhazen copied other teachings about optics (some correct and some wrong) from the Greeks and Romans.
end of synopsis > :
the camera obscura.
The camera obscura (pinhole camera) had originally been invented in 5th century BC China by the Mohist philosopher Mozi (墨子).
According to Wikipedia, both Mozi and Euclid stated that the inverted image in a pinhole camera demonstrated that light travels in straight lines from its source.
In the 6th century, the Byzantine-Greek mathematician and architect Anthemius of Tralles used a camera obscura in his experiments.
Alhazen learned about the pinhole camera (camera obscura) from reading Aristotle. http://www.mlahanas.de/Greeks/Optics.htm
All of the scholars using a camera obscura probably discovered the same 2 things: light travels in a straight line, and the image is turned upside down.
The Greeks actually did more research on the pinhole camera effect than Alhazen did, because the Greeks, in addition to creating pinhole cameras, studied how the effect was produced in a forest where leaves on trees blocked all direct light except for a few “pinholes” where no leaf was in the path of the light.
GREEK AND ROMAN OPTICS
Plato of 400 BC Athens who wrote about reflections in concave mirrors. It was translated into Arabic in the 9th century.
Among the works translated into Arabic in the 9th century Diocles in about 200 BC, who wrote about parabolas, burning lenses, spherical aberration and the mathematics of parabolic reflecting mirrors.
AL-KINDI EXAMINES GREEK THEORIES OF LIGHT
Al-Kindi (Alkindus) (801-873), living in Baghdad, rejected the tactile theory of Plato that an object sends a 3-dimensional copy of itself to the eye, because that would not explain why we only see the side that is facing us. Al-Kindi accepted the theory of most Greek philosophers that light must originate in the eye and bounce off the objects we see.
Al-Kindi (but not Aristotle) wrote that color of objects was the result of some colors of light being blocked, which is similar to the modern concept of some wavelengths being absorbed.
INCORRECT GREEK THEORIES OF LIGHT
Ptolemy and most Greek philosophers believed that a light beam originated from the eye, bounced off the front of an object, and then returned to the eye. [Plato’s tactile theory instead taught that the object sent a 3-dimensional copy of itself to the eye.] Nearly all of them believed light traveled at a finite speed.
ALHAZEN COPIED FROM EUCLID’S THEORY OF LIGHT
Alhazen’s theory of light was identical to that in “Optica” [written by Euclid in about 300 BC]; Light travels in straight lines, and sight is the result of light reflecting off objects to reach our eyes.
Even Alhazen’s “experiment” [that we see the stars immediately when we shut and later open our eyes at night, so therefore either our eyes do not emit light, or light travels at infinite speed] is copied from Euclid’s “Optica”.
See the paragraph about Euclid in the historical theories section of http://en.wikipedia.org/wiki/Light
Al-Kindi was not the first to write about the optics of perspective. Plato in the 4th century BC had described the mathematical rules of perspective.
Magnifying lenses were in use in ancient times, and a museum in Naples has an engraver’s magnifying lens unearthed in Pompeii.
In what is now Córdoba Spain, (Armen Firman) Abbas ibn Firnas [en.wiki] (810 – 887) manufactured magnifying lenses.
Ibn Firnas did not manufacture any spectacles.
In 984 AD a Jewish convert to Islam living in Baghdad, named ibn Sahl (ibn Sahl of Seville) (940 – 1000) published his results of repeating the refraction experiments described in Ptolemy’s book. Ibn Sahl discovered that Ptolemy was wrong in claiming that the angle of refraction was proportional to the angle of incidence. This is approximately true only for small angles. Ibn Sahl corrected this to say that the sine of the angle of incidence equals the index of refraction multiplied by the sine of the angle of refraction. This is Snell’s Law, which was not re-discovered by Europeans until about 600 years later.
Using Snell’s Law, ibn Sahl was the first person to calculate the mathematics of refraction of light passing through a hyperbolic lens.
In addition to his discovery of Snell’s law, Ibn Sahl, who was likely familiar with the writings of Diocles about the theory of parabolas, burning lenses, parabolic reflecting mirrors and spherical aberration, may have written about the theory of using parabolic lenses to correct spherical aberration. He did not create any parabolic lenses.
Alhazen (Ibn al-Haytham) (965 – 1040) patronized by the Shiite Sevener rulers of Egypt, made a number of discoveries in optics. Alhazen repeated the refraction experiment of Ptolemy, and rediscovered the same error Ibn Sahl had discovered, but instead of discovering Snell’s law merely created a table of how much the light was bent at various angles of incidence.
Alhazen mostly repeated what Ptolemy had written about visual perception. Alhazen repeated that the images from the 2 eyes are fused together. [Alhazen may have added that the eyes automatically move together to track a moving object, which helps to fuse the 2 images together, and the brain is even able to fuse the 2 images if the 2 eyes are not precisely pointed in the same direction, and Alhazen better explained why the images do not fuse if the object is very close to the eyes or off to one side.
Alhazen repeated what Ptolemy wrote about some process we are not consciously aware of automatically adjusting our perception of an object’s color under different lighting conditions, and adjusting our perception of the size of an object based upon how distant it is, and the brain sometimes making assumptions that can cause optical illusions.
ALHAZEN DID NOT KNOW HOW THE EYE WORKS
The Muslim claim is false that Alhazen applied the laws of refraction to explain how the eye worked. Actually, Alhazen agreed with Galen of Pergamon that the eye did not contain a lens, because if it did we would see things upside down. Alhazen’s explanation of how the eye worked was copied from Galen of Pergamon, both of them rejecting the idea that the image might be projected on the back of the eye.
Both Galen of Pergamon and Alhazen incorrectly thought that only rays exactly perpendicular to the eye were detected, and that all other rays of light entering the eye were ignored.
Alhazen mistakenly thought that the optic nerve was hollow and extended all the way through the eye to connect to what we now know is a lens (which he called the crystalline humor), and Alhazen thought the retina at the back of the eye played almost no role in vision.
WHY IS THE SKY BLUE?
Al-Kindi incorrectly wrote that the sky is blue because air has color.
Alhazen stated that the sky is blue because something in the air is blue in color.
Actually air “Rayleigh scatters” blue light and allows the other colors to pass through.
WHY IS THE SUNSET RED?
Alhazen realized that sunlight travels through more air at sunset, but his explanation that the sun is red at sunset because light was weakened by its long travel was not correct. Actually, it is because red light waves are scattered less by atmospheric gas molecules than are light waves of other colors in the visible range.
REFLECTION OF LIGHT OFF A SPHERICAL MIRROR
Alhazen proposed and solved the problem in 3 dimensions of given a light source and a spherical mirror, find the point on the mirror where the light will be reflected to reach your eye. [Greeks had earlier solved the problem in 2 dimensions of light reflected off a circle.]
ALHAZEN COPIED REFRACTION FROM PTOLEMY
Alhazen borrowed from Ptolemy’s 5th volume of “Optics” that light was refracted (bent) because light could not travel as fast through glass as it could through air. This is similar to the modern “line of soldiers” explanation of light being bent when it travels slower through glass than through air.
Alhazen was not the first to write about halos. Greek and Roman philosophers had given various explanations of halos being caused by reflection, refraction, or by rays of light hitting and compressing the air.
WHY DOES THE SUN LOOK LARGER ON THE HORIZON?
Alhazen was correct when he wrote that the sun and moon appearing larger when on the horizon is only an illusion in our mind, and not caused by refraction, but Cleomedes said the same thing at least 500 years earlier.
ALHAZEN WAS NOT THE FIRST SCHOLAR TO PERFORM EXPERIMENTS
Alhazen mostly repeated experiments that had been written about by Galen of Pergamon, and by Ptolemy (both from the 2nd century in the Roman colony of Alexandria), so it is false to claim Alhazen was “the first man to shift physics from a philosophical activity to an experimental one”.
ALHAZEN COPIED HIS THEORY OF LENSES AND MIRRORS FROM DIOCLES
Diocles in about 200 BC wrote about parabolas, burning lenses, spherical aberration and the mathematics of parabolic reflecting mirrors long before Alhazen did.
Alhazen wrote about the theory of parabolic lenses but did not create any parabolic lenses.
SEPARATING LIGHT INTO COLORS
In the 1st century, Seneca the Younger wrote that light passing through cylindrical glass rods shows colors.
Ptolemy wrote about reflection, refraction and color before Alhazen did.
AIR IS LESS DENSE AT HIGHER ALTITUDE
Alhazen copied from Ptolemy the idea that air was less dense at higher altitude.
A book written by either Alhazen or Abu ‘Abd Allah Muhammad ibn Ma’udh developed a relation between air density in the atmosphere and elevation.
COMPUTING HEIGHT OF THE GLOWING SUNSET
A book written by either Alhazen or Abu ‘Abd Allah Muhammad ibn Ma’udh used simple geometry to calculate the altitude of whatever in the Earth’s atmosphere glowed from sunlight hitting it after sunset (by timing how long twilight lasted to calculate the sun was 18 or 19 degrees below the horizon when the sky became completely dark).
REFRACTION BY ATMOSPHERE
Elsewhere in the short book, the phenomenon (known to the Greeks) of being able to see a star that should be up to six-tenths of a degree below the horizon, was correctly explained as being caused by refraction (bending) of light caused by differences in density of air at different altitudes.
Image by Fizyka z 1910, via Wikimedia Commons.
image credit https://commons.wikimedia.org/wiki/File:Camera_obscura_1.jpg