Fresnellenspronunciation

CAUTION: IT IS EXTREMELY IMPORTANT THAT EVERYONE WEARS SAFETY GOGGLES WITH THIS DEMO. Be sure to have extra people to ensure everyone (including volunteers!) wears safety goggles and does not touch anything that has been melted.

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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 collects light.

Fresnellenslighthouse

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. In this demo, we turn that same lens around to collect and focus sun light into a spot that 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

In optics, the numerical aperture(NA) characterizes the range of angles over which the system can accept or emit light. When the light hits a surface which is highly sloped, it can be reflected to not return up the optical system, and nothing will be detected. This means that higher NA objectives can measure steeper slopes.

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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 our 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.

Author: Mark Fedkin, John A. Dutton e-Education Institute, College of Earth and Mineral Sciences, Penn State University.

The Field of View (FoV) is the diameter of the circle of light that you see when looking into a microscope. The higher your magnification, the smaller the microscope field of view will be.

The John A. Dutton Institute for Teaching and Learning Excellence is the learning design unit of the College of Earth and Mineral Sciences at The Pennsylvania State University.