How did Archimedes use mirrors to burn up invaders' ships?
Category: Physics Published: December 11, 2012
Archimedes did not use mirrors to burn up the Roman invaders' ships. This myth claims that by focusing the sun's rays, Archimedes' mirrors raised the temperature of the ships enough that they caught fire. While a concave mirror held in one hand can light a paper in the other hand by focusing sunlight, burning a distant wooden ship would require an impractically large mirror. As published in the European Journal of Physics under the title "Reflections on the ‘Burning Mirrors of Archimedes'," the scientists A. A. Mills and R. Clift calculate that 440 flat square meter mirrors would be needed to smolder wood at a distance of 50 meters. Even then, smoldering wood can be extinguished easy with a splash of salt water. It would be impractical for the Greeks to produce, perfectly align, and effectively use 440 mirrors in 214 BC.
The problem is three-fold. First, ships that are far enough away to safely assault with sunlight would be too far away to reliably focus the sunlight. Secondly, ships are made out of sea-worthy wood which is much harder to light than paper. Third, sunlight only carries so much energy, even if perfectly focused. It takes a very large effective mirror to focus the sunlight to useful intensities. For example, the Sierra SunTower solar power plant uses 24,000 flat mirrors spread over 20 acres in order to focus sunlight onto two boilers. The focused sunlight at the SunTower is used to boil water, and then the resulting steam is used to turn electrical generators. Even though Archimedes' soldiers could not have used mirrors to burn the Romans' ships, this myth may have some basis in fact. Perhaps the mirrors were used to blind, confuse, or even burn the on-board Romans.