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Destroying Ships By Focusing Sunlight Through Mirrors
Could
Archimedes Really Destroy Ships By Focusing Sunlight Through Mirrors?
Ashish
I’m sure that many of us must have burnt a
piece of paper at least once in our lives by focusing sunlight through a lens
on a specific spot.
The feeling of seeing the first hints of a
black spot appear on the paper is bizarre, but very satisfying!
After succeeding in burning a small piece of
paper, many people have even tried to burn comparatively larger things by using
the same principle, but found little or no success.
However, rest assured, many others have tried
to do the exact same thing.
In fact, legend has it that Archimedes, the
famous Greek scientist and mathematician, also thought of causing destruction
using the same principle, only on an exponentially bigger scale.
It’s said that he created a ‘death ray’ by
focusing sunlight through a large number of mirrors (or a single enormous
mirror) onto Roman ships that were attempting to invade Greece.
Legend has it that he burnt down those enemy
ships using the death ray and sent the enemy army into complete disarray.
However, for this post, we’re not
particularly interested in the latter part of the legend; rather, we would like
to know if Archimedes ever actually built such a formidable ‘death ray’.
More importantly, is it actually possible to
burn ships by focusing sunlight on them through mirror(s)?
A Mirror’s ‘Burning’ Potential
Let’s return to our little ‘paper-burning’
experiment using a small lens.
Sunlight, or light rays coming from the sun,
carry a specific amount of energy.
Under normal conditions, sunlight doesn’t
cause paper to light on fire because the light rays are diffused, or too far
away from each other to be able to make any significant visible impact on the
paper.
However, when you position a magnifying glass
between the piece of paper and the light rays coming toward it, you essentially
cause those rays to come closer together and converge onto a specific spot
(called a ‘focal point’), which first turns black, and then, sometimes quite
dramatically, bursts into flames!
Can the same principle be used to burn
something bigger, say, a ship?
Theoretically, yes… you CAN burn larger stuff
(only things that can be burned, that is) using this principle, which means
that there is some theoretical basis to Archimedes’ ship-burning legend.
That being said, in a real-life situation,
it’s difficult enough to smolder a piece of wood, let alone set a huge ship
ablaze!
There are some practically insurmountable difficulties
in burning a ship using this technique. Let’s take a look at a few:
Size of the Mirror
The first thing that would have upset
Archimedes must have been the need to have a mirror; and a huge one at that!
You see, it’s easy to burn a hole in a flimsy
piece of paper with a small magnifying glass, but when it comes to burning a
life-sized ship from a considerable distance, or even cause the tiniest of
flames, you would need an impractically huge mirror.
Let’s suppose, for the sake of argument, that
Archimedes had access to such a huge mirror. He would then have to figure out a
way to transport, maneuver and point it precisely at the target without
significant difficulty and as swiftly as possible.
On top of that, Archimedes surely wouldn’t
want to crack or dent the mirror even a little bit while moving it, would he?
Using a Cluster of Mirrors?
Let’s assume that Archimedes used a cluster
of smaller mirrors, rather than using one giant mirror. Now, is there anything
wrong with that?
Yes… a lot of things, actually.
Scientists A. A. Mills and R. Clift
investigated this historical legend and published a study called ‘Reflections
of the Burning Mirrors of Archimedes’ in the European Journal of Physics.
They found that it took 440 men, each holding
a 1m2 mirror, to just begin to ignite a tiny area of 1*0.5 m of a wooden hull
placed at a distance of 50 meters.
This means that if you don’t want to try a
giant mirror, you would need at least 440 mirrors and just as many men to aim
them precisely at the target.
Even after all that effort, you would be far
from obtaining the desired result.
Supposing you get those 440 men to aim at a
spot with surgical precision, you would still only be able to create a minor
smolder, which could be easily quenched by saltwater (which is readily
available to soldiers onboard a ship).
Also, do you think that the enemy ship would
be stationary and simply wait for your array of mirrors to inflict any serious
damage on it?
Mythbusters, a popular Discovery Channel
show, also tested if a ship could be burnt just by using mirrors, and concluded
that first, it’s an uphill task to focus multiple mirrors’ reflected beams at a
single spot.
While you could achieve a temperature of a
couple hundred degrees, it still won’t be as hot and as powerful as to be able
to burn an entire ship down.
All in all, it’s practically impossible to
burn a ship down by focusing sunlight on it through the use of mirrors.
However, you can certainly blind, puzzle or
inflict some burns on the skin of certain crew members onboard the enemy ship.
Turns out that Archimedes’ idea for this sort
of assault on an enemy doesn’t hold much water, but history is always told by
the victors!
Ashish is a
Science graduate (Bachelor of Science) from Punjabi University (India). He
spends a lot of time watching movies, and an awful lot more time discussing
them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly
Science dictates every aspect of life… in this universe, at least.
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