During the siege of Syracuse in 212 B.C., Archimedes supposedly used mirrors to concentrate the sun’s rays on enemy Roman ships, setting them on fire. This legend has been intriguing people for centuries. Is it a myth? Or is it feasible to ignite ships using mirrors? Would Archimedes have had the know-how? The consensus of the research I’ve done is that, while it is certainly possible set fire to an object using only mirrors, carefully controlled conditions are necessary. Archimedes most likely had enough knowledge of mathematics and optics to design such a weapon; however, it is highly improbable that the death ray would have been a possibility during the heat of battle. Furthermore, there were several other known methods to set fire to enemy ships that would have been better choices.
Flat mirrors will not focus sunlight onto one small spot. In order to create the necessary intensity of light, a group of flat mirrors arranged in a parabolic shape OR a single large mirror in the shape of a parabola would be necessary. Scholars believe Archimedes would have been more apt to use a collection of small mirrors held by soldiers, because a very large area is vital to channeling enough sunlight to initiate combustion. A single mirror with this area would be impossibly unwieldy. Parabolic shapes have a special mathematical property called a focal point, which arises from their curved shape. In the case of a parabola made of mirrors, this means that each individual small mirror will focus parallel rays of light on one particular point – the focal point. The distance of the focal point from the mirrors is called the focal distance, and it depends on various factors. Light, and thus, heat, are most intense at the focal point, which can be calculated mathematically. Light is more spread out at points that are closer to the mirror, and at points that are farther away from the mirror, which means the temperature is not as great at these locations.
This sounds fantastic in theory, but adjusting the distance of the mirrors so that the focal point is centered on a single ship would be an enormous challenge during battle, when ships are in motion. A related problem concerns the length of time needed to set wood on fire. In modern recreations, using modern mirrors superior to the polished copper or bronze available to Archimedes, ignition has not been instantaneous. Depending on the size of the parabola, the distance from the target, the material it is made of, and other factors, at least ten minutes have been necessary for ignition.
Setting enemy ships on fire was a standard tactic in the time of Archimedes. Other methods, such as catapulting a fiery mixture of incendiary materials onto ships’ decks, were more practical, reliable, and efficient. Historians did not mention the death ray until about 800 years after it was supposedly invented. Most tellingly, there is no evidence that the Romans ever adopted the death ray as a weapon. If it was truly as devastating as is claimed, then they would surely have figured out how it worked and added it to their arsenal.
The death ray debate is still far from over! MIT and Mythbusters are two of more prominent groups that have simulated the death ray in recent years. The MIT students were successful in igniting their ship -- the Mythbusters were not!
Claus, A. C., "On Archimedes' burning glass," Applied Optics, 12, A14 (October 1973)
Claus is convinced that the death ray was not beyond the ancient Greeks’ capabilities. This civilization was familiar with the laws of specular reflection, and they were known to use pieces of polished metal as mirrors. However, the manner in which Claus believes the weapon was used is rather elaborate. According to his theory, several hundred people would need to focus mirrors at the same time so that the light was concentrated in one location. They would probably have to stand together on a hill, and would have needed training to focus their individual mirrors to collectively ignite an object in this way.
Simms, D. L., "Galen on Archimedes: burning mirror or burning pitch," Technology and Culture, 18, 91-96 (1991)
Simms reviews the original passage in the historian Galen’s De Tempermentis that has been used as evidence for and against the death ray. The debate centers on whether the reference to Archimedes’ weapon meant a “burning mirror” or a “fire pit”, which would have been better known at the time. This article was useful not only because it pointed up alternative possibilities for the means by which the ships were set ablaze, but also because it cast a bright light on the historical record. No mention of a burning mirror was made until 800 years after the event supposedly took place. I consider this to be a reliable source because it is a respected journal.
Stavroudis, O. N., (letter) "Comments on: on Archimedes' burning glass," Applied Optics, 12, A16 (October 1973)
Stavroudis dismisses the idea that Archimedes could have destroyed enemy ships with mirrors as farcical. He asserts that the original notion of a “burning mirror” came from Aristophenes’ play “The Clouds”, in which a character fantasizes about using this tactic to destroy a warrant for his arrest. The instrument's focal length would have to have been long enough to keep it and its operator out of harm's way; yet variable, to allow ranging and tracking; and with an area large enough to collect enough energy to do the job. Given the large number of people required to operate such mirrors, and the rarity of mirrors at the time of Archimedes, it is unlikely that this is the weapon mentioned by historians. Stavroudis believes the weapon was a mixture of sulfur, pitch, charcoal, incense, and tallow which was catapulted onto the decks of enemy ships. This article was useful because it introduced the idea that other possible weapons could be confused with the burning mirror phenomenon. I consider it to be reliable, as it is from a respected journal.
MIT Feasibility Test
This website provided detailed information about the testing carried out by students and professors at MIT, both independently and in combination with Mythbusters. The many pictures on the site were especially helpful to me (some are included in this page), and I trust this website because MIT is a prestigious university.
Archimedes and the Burning Mirror
This website helped me to understand the math behind focal points and focal distances. It also included an overview of the MIT and Mythbusters recreations of the death ray, along with some of the pictures I included in this page. Although the information was useful to me and jibed with my other sources, there was heavy advertising and I had never heard of the Museum of Unnatural Mystery before!