POPtics Fresnel Lens

Overview: A Fresnel lens can be looked at as many layers of lenses cut and stacked on top of each other. Creating a lens in this manner provides great collecting or great dispersion power. Lighthouses use Fresnel lenses to project light miles out into the ocean. If you turn that same lens around to collect and focus sun light into a spot, it is so powerful that it will melt rocks. While Fresnel lenses can collect a lot of light in a compact lightweight package, they do not have very good resolution.

​​Supplies: Fresnel lens card, protective eyewear

Objectives: Learn more about a type of lens structure that can be lightweight, for its size, yet very powerful at the same time for the way it magnifies and collects light.

Setup:

  • Touch each side of the Fresnel lens. There will be a smooth side and a ridged side.

How to run the demo:

  • To use the Fresnel lens as a magnifier, place the smooth side up and place the card a distance from an object where it is in focus and magnified.
  • Move the card up and down until you get the desired magnification.

Want to try to melt something?

  • PUT ON YOUR TINTED SAFETY GLASSES
  • Flip the card over so the ridged side is facing up. Take the card out into the sun. On a bright day you should see a bright focal spot underneath the lens.
  • Since the area of the lens is very small, place an object that is easily burned or melted on the ground
  • Bring the Fresnel lens down and move the focal point to the surface of the object you are trying to melt.
  • After several seconds, you should see some smoke as the object melts!
  • Put the Fresnel lens down and wait several minutes for the object to cool before touching it.
  • Tip: Use a bottle of water to cool off your object!

What’s Happening?

A Fresnel lens replaces a traditional curved lens by adding concentric grooves to the surface of the lens, as seen below left. These grooves act as individual refracting surfaces that turn a light source into parallel rays that can be seen for very long distances, as in the case of the lighthouse. In the second part of the demo we take the light in the opposite direction. We face those grooves toward the sun, and its incoming parallel rays, to concentrate the light down to a single point. Fresnel lenses are mostly used for light gathering power not for precision imaging.

Try this:

Imagine a magnetic board and lens shaped magnets to demonstrate how the shape of a Fresnel lens works.