The Fresnel Lens
A Fresnel (pronounced Fra-NEL) lens is what makes the small light at the top of a lighthouse into a strong one that can be seen for many miles out to sea. The Fresnel lens was invented almost 200 years ago and was such a good design that it is still being used in many lighthouses.
The lens is named after its inventor Augustin-Jean Fresnel, who was born in France in 1788. Fresnel had problems in school. He did not like to pay attention and memorize things. But he was very good in math and science and won a contest that helped him get into a famous military school.
Prisms and Light
After he graduated, Fresnel went to work as an engineer building bridges and roads all over the country of France. In his spare time he loved to do experiments with light. He discovered that prisms could bend light and lenses could condense that light. It seemed to him that this combination would work well in lighthouses. A group of prisms could be used to bend the light coming from a lamp, and big lenses could be used to focus the light into a single strong beam.
Fresnel lenses come in different sizes. The largest size is the first order lens, and you could easily fit inside one. The next sizes get smaller and are called second order, third order large, third order small, fourth order large and fourth order small, fifth order, and the smallest or sixth order lens. By 1835, most of the lighthouses in Europe were using Fresnel lenses.
An experiment for you to try
If you would like to try one of Fresnel's experiments with light and lenses here is what you will need:
This is what to do:
- A small piece of stiff white paper like a file card
- A hole puncher
- A small drinking straw
- A scrap of printing from the newspaper or an old magazine
- Two small amounts of honey - make sure one is room temperature and the other is cold from the refrigerator
Punch two holes in the file card, not too close to each other. Hold the file card about 10" above the scrap of printing and look at the printing through one hole to see what happens.
What to notice:
- Cut a 3" section off the straw and cut it on the diagonal.
- Use the diagonal end of the straw to dip a drop of cold honey from its dish.
- Cover one hole on the file card with the drop of cold honey.
- Look through the honey-covered hole at the scrap of printing.
- Cover the second hole with a drop of room temperature honey and look through the hole at the same printing.
- When you look through the hole before it is covered with honey, the printing might look a little bigger because the hole helps focus your vision.
- The honey will work like a lens and make the printing look larger or smaller.
- If the honey is warm and sags down through the hole, it will form a convex lens (the lens is thicker at the center than at the edges of the hole) and make the printing look bigger.
- If the honey is cold and forms a lump on top of the hole, it will form a concave lens (one that is thicker at the edges of the hole rather than at the center) and make the printing look smaller.
- Try holding the file card at different distances from the printing. Does this make a difference in what you observe?
- What happens if you change the size of the holes in the paper?