Friday, January 15, 2010

The Ancient History of Optics

People have puzzled and pondered over how we see for many thousands of years, though a more complete understanding of optics was not truly available to us until more recent centuries through the work of scientists like Newton, Young, Maxwell and Dirac. Some of the earliest known writings on optics date back to the Ancient Greeks. The majority of the earliest ideas on optics were largely speculation. In the fifth century BC for example, the greek philosopher Empedocles hypothesized that the eye contained fire, which shone out from the eye illuminating objects so that they could be seen. Of course this raises many questions, such as "why can't we see at night then" and "why don't objects appear incredibly bright when several people look at them" - for the former question Empedocles thought that there may be some interaction between the rays from the eyes, and rays from a source like the Sun or a candle. Unfortunately the concept of Occam's Razor (entia non sunt multiplicanda praeter necessitatem - or entities must not be multiplied beyond necessity) wasn't devised until the 14th century, or Empedocles' contemporaries may have suggested doing away with the eye-beams and just leaving the rays from bright objects like the Sun. Still, it was relatively early days, and there were many great thinkers to come, who would apply their minds to the problem of light and vision.

To backtrack a little here, while there was no theoretical understanding of light and optics, People had still made practical applications of optical components and light itself. One of the earliest known lenses for example was the Nimrud lens, shown here - a piece of shaped glass some 3000 years old in the remains of the ancient city of Nimrud, which lies within modern-day Iraq. This lens was discovered by Austen Henry Layard in the mid 19th century, and may have been used as a magnifying lens, either for looking at objects or starting fires. Many similar lenses exist through the ancient Greek, Roman, Babylonian and Egyptian cultures. These though tended to be carved from crystal or glass spheres filled with water.

Back on to the theory. A couple of hundred years after Empedocles speculated on how vision worked, Euclid wrote Optics, which was one of the first texts to study the geometrics of optical systems in more mathematical detail. Ptolemy, this time a Roman Citizen (though possibly of greek ancestry) who lived between the first and second centuries BC, extended this work futher. His writings are considered some of the most important writings on optics before Newton, although they survive only as translations into Arabic - the originals having been lost. In this, he introduces many of the important properties of optical systems, talking about light, refraction, reflection and colour (all things I will get on to later)

Not all of these early studies were limited to the Greek and Roman empires though, much important early work was also carried out by Arabic scholars such as Ibn Sahl (10th century BC) who discovered the law of refraction (now known as Snell's law) and Ibn Al-Haytham (10th-11th century BC) who did away with Empedocles' rays from the eyes and more carefully defined what the rays were.

Things lay relatively quiet on the optics front then, until the Rennaisance when scientists such as Johannes Kepler and Willebrord Snellius began investigating the mathematical and physical behaviour of light. That though, will have to wait until later.


  1. Do you know why Ibn Sahl was not given credit for discovering the law of refraction? Why was W. Snell given credit and the refraction law named after him?


  2. I don't know for sure, however much like many other scientific and mathematical discoveries like pythagoras' theorem, it may be that Snell's law was discovered multiple times, with Ibn Sahl's discoveries having been forgotten for many years, and rediscovered after Snell's name was attached to this law.