lighthouses and Fresnel Lens

Ponce Inlet lighthouse, Florida

Ponce Inlet Lighthouse – Florida’s 1350 miles of coastline with its lighthouses played a critical role in the exploration, settlement and development of Florida’s history. Ponce de Leon Inlet lighthouse was built in 1887 and is the only Florida lighthouse listed as a National Historic Landmark and is the second tallest brick tower in the United States. The lighthouse beacon was electrified in 1933 with a 500-watt lamp. The first order Fresnel lens was replaced with a third order rotating Fresnel lens at the same time.

Other lighthouses

  • Point Reyes, CA – Point Reyes is the windiest place on the Pacific Coast and the second foggiest place on the North American continent. Weeks of fog, especially during the summer months, frequently reduce visibility to hundreds of feet. The Point Reyes Headlands, which jut 10 miles out to sea, pose a threat to each ship entering or leaving San Francisco Bay. The historic Point Reyes Lighthouse, build in 1870, warned mariners of danger for more than a hundred years.
Fresnel Lenses

Fresnel lens (pronounced /freɪˈnɛl/ fray-nel or /ˈfrɛznəl/ frez-nel) – a type of compact lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses. The first Fresnel lens was used in 1823. The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances. Fresnel produced six sizes of lighthouse lenses, divided into four orders based on their size and focal length.

Picture: The original first-order Fresnel lenses of the Cape Canaveral Lighthouse (right) and the Ponce de Leon Inlet Light (left) on display at the lighthouse museum of the Ponce de Leon Inlet Light Station. The original light was fueled by kerosene and could be seen for more than 18 miles out in the ocean because it was magnified by a special Fresnel lens.

What’s the problem?

Spiral staircase inside the lighthouse

In order to see a light on shore from a long way at sea, the light source needs to be as high as possible.

  • Ask – How high should the light be? How can the light be supported? What kinds of weather conditions does the lighthouse structure have to withstand?
  • Imagine – Most of the lighthouses were built more than 100 years ago. Bricks, wood and metal were common building materials. The light could be positioned at the top of a tall cylinder or square building.
  • Design, Build – An internal spiral staircase could be used to reach the light in any weather. Large objects could be lifted up the space in the center.
  • Improve – The kerosene lanterns were replaced with electric lights that could be operated from the ground, so the lighthouse keeper didn’t have to carry fuel up to the lantern throughout the night to keep it lighted.

Do it
Here are some challenges for you to work on…

  • calculate the height of a lighthouse needed to be seen 50 miles away on the ocean. You need to know about the curvature of the earth for this.

Learn more…