Fresnel Lenses
by Chris Woodford. Last updated: May 18, 2011.
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!
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. A hi-res version of this image is available for commercial use from our Flickr page.
How a lighthouse works
Photos: This is Anvil Point lighthouse near Swanage in Dorset, England. It was originally built in 1881 and fully automated over a century later in 1991.
A lighthouse uses similar science to a telescope, but works in exactly the opposite way. 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!
Three cunning tricks make this possible:
- Lighthouses use amazingly powerful xenon lamps (a little bit like neon lamps) that are hundreds of thousands of times brighter than the lamps in your home.
- They mount their lamps in towers high above the sea level, which makes them visible roughly five times further away.
- They use specially shaped lenses and prisms to concentrate their light into a super-powerful beam.
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.
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.
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.
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.
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.
Further reading
On this site
On other sites
- The Fresnel Lens: An excellent 5-minute video by Jonathan Hare of the Vega Science Trust, who explains with great clarity why lighthouses need lenses and why Fresnel lenses do the job best.
- A lighthouse keeper's tale: BBC News, 12 May 2010. Many people dream about the peaceful solitude of being a lighthouse keeper, but what's life like on a lighthouse in reality?
- Augustin Fresnel: Biography: A brief account of Fresnel's life and his research into how light waves behave.
Books
- DK Eyewitness: Light by David Burnie. Dorling Kindersley, 1998. A good introduction to the history of light science from the excellent DK Eyewitness series. Ages 9–12.
