How it Works: The Fresnel Lens

Some years back, a magazine ad for something called the Edmund 
Scientific’s Giant Fresnel Lens 
ran a headline that boasted, Melts 
Asphalt in Seconds! One can assume 
that this was not exactly what French physicist Augustine Fresnel (pronounced Fre-NELL) had in mind.
In 1822 Fresnel invented the most important breakthrough in lighthouse lights in two thousand years. Fresnel’s beehive-shaped system of lenses and prisms transformed a single oil-burning lamp into a powerful beacon that could be seen twenty miles out to sea.
However, as much of a breakthrough as the Fresnel lens was, it would take several years for the United States to see the light. At a cost of $12,000 per lens, it was considered a tad pricey.
The turning point came when the shipping industry essentially blackmailed the U.S. government by saying, If you can’t give us lights to guide us into your ports, we’re not coming into your ports.
So there.
The Gay Head Light was one of the first American lighthouses to acquire the Fresnel lens. It guided mariners up and down the busy coastal waterway of Vineyard Sound from 1856 to 1952.
In theory, using a Fresnel lens is like holding a magnifying glass up to a light. But in place of a single lens, there are over one thousand prisms and round “bull’s- eye” lenses that bend the light into a powerful beam. A First-Order Fresnel lens (the largest type) such as the one at Gay Head weighs over two tons.
On the circumference of the lens, each prism is identical to the one next to it but differs from the ones above and below. One can only imagine the amount of hand labor that went into creating this instrument. In addition, there are ruby-colored glass panels positioned around the circumference that cause a red flash as the huge lens assembly rotates around the light. This gives each individual lighthouse its identifying flash or “characteristic.” In the case of the original Gay Head Light, three whites and one red.  
The Gay Head Fresnel lens now 
resides at the Martha’s Vineyard 
Historical Society in the heart of Edgartown. Curator Jill Bouck attests to the power of the lens and notes that with 
just a 120-watt bulb, it can illuminate 
the entire neighborhood. In daylight hours, she says, the lens must be shielded from direct sunlight, otherwise the 
sun’s rays become magnified. Several years ago, a workman left a wooden 
ladder inside the lens and when the sun came up the next day, the ladder nearly caught fire.
Today’s lenses tend to be made of plastic, and the old whale-oil lanterns have been replaced with 12-volt lamps. These newer versions generally don’t hold a candle to the old Fresnel lens. But with the advent of such things as LORAN and satellite navigation, a twenty-mile beam is not necessarily required. And while I have no scientific proof of this, I’ll bet they’re not nearly as good at melting 
asphalt either.