A lighthouse uses similar science to a telescope, but works in exactly the opposite way—with the help of a Fresnel lens. The glass lenses in a telescope refract (bend) light rays from distant objects so they seem to be much nearer. But in a lighthouse, the Fresnel lens wrapped around the lamp concentrates the light rays into a powerful and parallel beam so people can see it, with just a naked eye, as far as 30 km (about 20 miles) away or more!

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In theory, you could make a lighthouse beam with just an ordinary glass lens, but it would need to be enormous and heavy and that would make it incredibly expensive and quite impractical. That's why lighthouses use hollow, lightweight Fresnel lenses, which have a very distinctive "stepped" surface that bends the light as much as a thick, heavy glass lens. They're named for Augustin-Jean Fresnel, (1788–1827), the French physicist who pioneered them in the early 19th century. Car headlamps use Fresnel lenses molded from plastic in much the same way and you'll also find them in some of the traffic signals (traffic lights) standing on your street.

Today, we remember Brewster for other optical innovations—notably his work on polarized light. Ultimately, it was Fresnel who figured out how to make a simple, practical, affordable, multi-part lens that could be used effectively in lighthouses—and that's why he gets the credit! Even so, as Margaret Gordon's (admittedly partial) book makes clear (from p.382 onward), numerous distinguished scientists regarded Brewster, not Buffon or Fresnel, as the true inventor of the built-up lens system deployed in lighthouses.

Fresnel lenses are all about making big, powerful beams of light that stretch very long distances. Unlike the conventional lenses in something like a telescope, the optical quality of the light beam emerging from a Fresnel lens often doesn't matter very much: if you're operating a lighthouse, all you're trying to do is throw light off into the distance; it's not important if sailors see a "blurred" image of the lighthouse... as long as they can see something! That means Fresnel lenses can be made from relatively inexpensive plastic, such as acrylic or polycarbonate, as well as glass. You simply need a mold containing the lens pattern in reverse—and then you can make as many identical Fresnel lenses as you want! Plastic Fresnel lenses are smaller, thinner, weigh less, and cost less than comparable glass lenses.

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But hang on just a moment, why does Fresnel get all the credit? He certainly pioneered this type of lens, in 1822/23, but the idea of cutting a piece of glass down to its useful, refracting, outer edge wasn't original. About a half century earlier, for example, Frenchman Georges de Buffon had come up with a very similar idea, which British optical physicist Sir David Brewster subsequently turned into an alternative lighthouse technology called the built-up or polyzonal lens (sometimes also called the "Holophote") around 1811. According to The Home Life of Sir David Brewster, a book published in 1870 by his daughter, Margaret Gordon:

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Artwork: How the elements of a Fresnel lens in a lighthouse bend incoming light rays by different amounts to make a parallel beam of outgoing light rays. Redrawn from a historic illustration [Adolphe Ganot (1872) Natural Philosophy for General Readers and Young Persons, D. Appleton & Co., New York, p.328, fig.257], courtesy of Wikimedia Commons.

You'll probably also find that different colors are refracted by the lens to different degrees, giving you unwanted color fringes in your image (a problem called chromatic aberration). Although it's possible to adjust the angle of the steps in a Fresnel lens to minimize aberrations, generally you'd use a conventional lens (probably made from optical quality glass) for higher optical performance.

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Fresnel lenses save wayward sailors from shipwrecks and death on the rocks, but could they save our whole planet from a climate catastrophe? As people become more aware of climate change and the enormous challenge it presents, some scientists think we will have no choice but to try one or more forms of geoengineering: technical fixes to cool the planet. One option that's been proposed is a giant Fresnel lens positioned between Earth and the Sun to diffuse the solar radiation and cool our planet. Though schemes like this are extremely controversial, they do illustrate the fascinating applications of Augustin-Jean Fresnel's brilliant ideas!

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It's hard to get close enough to the Fresnel lens in a lighthouse to see exactly what it's like, but if you're near a science museum you might just be lucky—they sometimes have old Fresnel lenses on show. Here are some photos I took of the working Fresnel lens at Think Tank, the science museum in Birmingham, England. Note how there is a Fresnel lens on each side of the lamp (making eight in total) with prisms (curved chunks of glass) mounted above and below the lens to pull in light rays that deviate further from the central axis, making an even brighter beam.

Photo: Sir David Brewster—original inventor of the lighthouse lens?—in an early (c.1844) photograph by Robert Adamson and David Hill. Photo courtesy of The Metropolitan Museum of Art.

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The lamp in this particular lighthouse rotates so it sweeps across a much greater area of the sea. The rotation also means the light seems to flash every 10 seconds when you're far away from it. That makes the lamp much more noticeable and, because different lighthouses flash at different rates, sailors can time the flashes to figure out which lighthouse they're looking at and where they are.

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Woodford, Chris. (2008/2022) Fresnel lenses. Retrieved from https://www.explainthatstuff.com/fresnel-lenses.html. [Accessed (Insert date here)]

If you're unlucky enough to be caught in a storm at sea, there's nothing more reassuring than the friendly wink from a nearby lighthouse. But have you ever stopped to think how that light can travel so far across the ocean? It's largely down to lenses—amazing, curved, Fresnel lenses (pronounced "Fre-nel," with a silent "s") that concentrate light into a super-powerful beam. Let's take a closer look at how they work!

Lenses work by bending (refracting) light beams. The bending happens when light enters the glass (passing from the air into the glass) and when it leaves again (passing from the glass back into the air). It follows that the only part of a lens that really matters is the border between the glass and the air (in other words, the outer edge of the lens); most of the lens doesn't really do that much at all. What if we could cut a lens right down so that all we had left was the useful outer part? That's the basic theory of the Fresnel lens—and here's how it works in practice.

Photos: A closeup of the Fresnel lens in the Anvil Point lighthouse. The 1080-candela beam fires out at about 45m (148ft) above sea level, and it can be spotted up to 17km (9 nautical miles) away.

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If you want to use a Fresnel lens the other way—to collect light rays from a distance and bring them into a sharply focused image—you need to be more precise. Inexpensively made Fresnel lenses make poorer quality images than traditional glass lenses because of a problem called spherical aberration: light rays traveling through a Fresnel lens at different angles will come to a focus at slightly different points, giving a blurred image. You get quite a bit of distortion because the surface of a Fresnel lens is discontinuous: unlike with a smoothly curving lens, there are sudden jumps from one segment of a Fresnel lens to the next.

Artwork: Lighthouses and car headlamps aren't the only places where you'll find Fresnel lenses. Projection TV systems sometimes use them to make large, magnified images. In this 1980s design by RCA, an image generator (green, 102, maybe a traditional CRT or LCD screen), fires its picture onto a back-projection screen via a prism and mirrors (blue, 103/104), through a compound Fresnel lens (red, 10) and focusing lens (9, pink). Artwork from US Patent 4,482,206: Rear projection television screen having a multi-surface Fresnel lens by Bertram VanBreemen, RCA, November 13, 1984, courtesy US Patent and Trademark Office.

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Photos: The Fresnel lens exhibit at Think Tank, the science museum in Birmingham, England. The silvery thing at the bottom is the electric turntable that makes the whole lamp and lens assembly rotate very slowly.

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Photo: If you see a light shining through a piece of glass or plastic with this weird pattern of concentric circles on its surface, you can be sure you're looking at a Fresnel lens.

Look closely at a lighthouse and you'll see the Fresnel lens surrounding the lamp. The concentric rings are actually "steps" (thick ridges) in the lens surface. Each step bends the light slightly more than the one beneath it, so the light rays all emerge in a perfect, parallel beam that travels many kilometers/miles across the ocean.

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Photo: Car headlamps often use inexpensive, plastic Fresnel lenses to throw light off into the distance. Although plastic Fresnel lenses are cruder and produce poorer quality images than traditional glass lenses, it doesn't matter in this case: all that's important is concentrating light into a beam at a reasonably well focused point on the road ahead.

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Photos: The Fresnel lens at Anvil Point lighthouse near Swanage in Dorset, England, which was originally built in 1881 and fully automated over a century later in 1991.

Consider for a moment what depth of field tells us. It is the distance by which an object may be shifted before an unacceptable blur is produced. For depth of ...