by Chris Woodford. Last updated: November 6, 2016.
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!
How does a lighthouse work?
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!
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.
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.
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.