MIRROR SETS A SHIP ON FIRE?

Click here to return to HOME page.
This post is in category PHYSICS

<><><> start of synopsis ><><><><><>

The front page illustration of a Latin translation of Alhazen’s “The Book of Optics”  depicts a 2nd century AD myth, repeated by Alhazen, falsely claiming Archimedes had invented burning lenses that set ships on fire.

<><><> end of synopsis ><><><><><>

931px-thesaurus_opticus_titelblatt

front page illustration of a Latin translation of Alhazen’s “The Book of Optics”.
It depicts a 2nd century AD myth, repeated by Alhazen, falsely claiming Archimedes built (parabolic?) mirrors in 212 BC that set the Roman fleet on fire.


The technology to build parabolic mirrors did not exist until the 17th or 18th century. [Precisely aligning a hundred flat mirrors joined together to form the approximate shape of a parabola would also have been beyond the limits of their technology.]
Besides, the weapon would be impractical for several reasons. The point at which the rays meet can only be in the direction exactly toward the sun, so the mirror would need to be at sea level, pointing upwards toward a nearby target ship, not pointing down from a hill as shown in the illustration. Even at anchor, the target ship would be in motion, limiting how long the hot spot can be focused on the same spot on the ship.


The parabolic mirror would need to have a focal length of hundreds of meters, so the parabolic mirror would need to be almost flat. An ordinary flat mirror could not be placed above a portion of the parabolic mirror to reflect the light to a different direction, because that would block the path of the rays coming to that portion of the parabolic mirror.

[Dish-shaped antennas and camping parabolic fire-starting mirrors do not have this problem because their short focal lengths allow the light to hit at a steep 30 degree or 45 degree angle, so the ray does not have a return path nearly identical to its entry path.]


Alhazen, like the Greeks who preceded him, built small spherical lenses, with a short focal length, that could set small objects on fire. Alhazen never claimed that he himself set any ships on fire.


Much of the information on Muslim websites about Alhazen is false, giving him credit for things actually copied from the Greeks, Romans and Chinese.


Image by unknown medieval illustrator, via Wikimedia Commons.
image credit https://commons.wikimedia.org/wiki/File:Thesaurus_opticus_Titelblatt.jpg#mw-jump-to-license

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s