What areFresnellights used for

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Nearly all lighthouses in National Park units originally had a Fresnel lens, though many of them have been removed and/or replaced with more modern lighting mechanisms. However, the lenses’ beauty and their pivotal place in lighthouse history has ensured their preservation in many instances. Some are in museums associated with the historic lighthouse itself; others are in museums away from the lighthouse. And of course, there are many, many more Fresnel lenses in American lighthouses that are not part of National Parks. The United States Lighthouse Society maintains a large amount of information about Fresnel lens history and technology, as well as lists of current and operational Fresnel lenses in the United States.

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The iPhone's magnifying feature is easy to activate, and it's a neat trick to hold up your sleeve in case you end up forgetting your glasses. Here's how to use it.

That's it. It's really useful, even for people who might wear glasses but still need help zooming into super small text, like the tiny text in the manual I've used in the picture above.

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The use of lenses in lighthouses began in England in the 18th century, and was adopted in the United States by 1810. These early lenses were thick, excessively heavy, and of poor quality glass. Therefore, they were not very effective and prone to losing the light through the thick glass. In 1811, the French Commission on Lighthouses established a committee to investigate improvements in lighthouse illumination. Among the committee members was Augustin Fresnel, who in 1822 completed the design of his flashing lens using thin bull’s eye shaped panels, which refracted the light both horizontally and vertically, producing a much stronger beam of light.

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Use the slider to zoom in or out of text. Tap on the screen to focus, much like you would with the camera app. Tap the lock button to lock the focus in place, or use the filter button (the one on the bottom right) to apply various filters that might make it easier for you to read. Finally, tap the center button to capture a picture.

Fresnel lenses may be fixed, showing a steady light all around the horizon, or revolving, producing a flash. The number of flashes per minute is determined by the number of flash panels and the speed at which the lens revolves. A unique flash pattern for each light is produced by varying the amount of light and dark periods. For example, a light can send out a flash regularly every five seconds. Alternatively, it might have a ten second period of darkness and a three second period of brightness, or any number of other combinations. The individual flashing pattern of each light is called its characteristic. Mariners consult a light list or a maritime chart that told what light flashes that particular characteristic, and what color the light is. This allowed them to determine their position at sea in relation to the land.

Here's how to use magnifier when you need it: If you're on using iPhone X, iPhone XS, iPhone XS Plus or iPhone XR, just triple tap the button on the right side of the phone. If you have an older iPhone, tap the home button three times to bring up the magnifying app. You'll see a screen that looks like this:

It's perfect if you're in a dark restaurant and forgot your glasses. While most folks seem to know about the flashlight option on the iPhone, the magnifying feature, which uses the phone's camera to zoom in on text, seems a little less well known. That's because it's buried in the settings menu.

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The Fresnel lens (pronounced "Frey Nel"), as it came to be known, represented a monumental step forward in lighthouse lighting technology, and therefore also in maritime safety. In a Fresnel lens, hundreds of pieces of specially cut glass surround a lamp bulb. This design intensifies the glow from the light, focusing rays of light that would normally scatter into a single, intense beam of light, which shines out in a specific direction. The lens could produce an unlimited number of flashing combinations and intensified the light so it could be seen at greater distances, allowing mariners a greater deal of safety in their navigations near shore.

Fresnel lenses came in several sizes, or orders, from the largest, the Hyper-Radial, to the smallest, the eighth order. Not all orders were used in the United States. Large first order lenses, such as those still in place at the Fowey Rocks lighthouse in Biscayne National Park or the Bodie Island lighthouse in Cape Hatteras National Seashore, were usually used on major seacoasts, with a more powerful beam that shines up to twenty-one miles out to sea. Fifth or sixth order lights, the smallest used in the United States, were used in smaller bodies of water, such as bays or rivers. The Jones Point lighthouse on the Potomac River in Alexandria, Virginia, used a fifth order lens for the comparatively smaller distances it had to cover, but it was nonetheless essential to the hundreds of merchant, passenger, fishing, and naval vessels that traveled the waters around Washington, DC daily.

Most Fresnel lenses look like a beehive or barrel; most contain from two to twenty-four different panels. A clock type mechanism, which had to be wound by hand every few hours before automation, was used to make the revolving lenses rotate around the lamp itself to produce the flash. The movement of the lens is timed precisely so the panel will pass by when a flash is due.