A Fresnel lens, left, is a type of magnifying glass which simulates the shape of a conventional lens using ridges. Conventional lenses require significantly more material and become quite heavy as they increase in size and power, particularly if they are made of glass. Even a very large fresnel lens will use only a a small fraction of the material and still be quite lightweight.

Fresnel lenses were originally developed for use in lighthouses to disperse light emenating from a single point (see diagram at right, and download powerpoint slides from the Smithsonian Museum of American History). By reversing the lens, incoming sunlight can be concentrated. With a fresnel lens, the total amount of light concentrated at the focal point depends on the available light, the size of the lens and the number of ridges. Our 33" x 44" fresnel lenses can concentrate enough sunlight on a clear day to reach 3,000° at the focal point!

Because light is being focused, heat is only produced if there is an object at the focal point to absorb the light. Air itself does not heat up significantly, and clear glass placed at the focal point will not melt since too much of the light simply passes through without being absorbed. Pure white objects will reflect most of the light away and also not absorb enough light to heat up significantly. For this reason, we cannot make Sundrops that are completely clear, white or extremely light colored. The lightest colored glass we use is recycled from Bombay Sapphire Gin bottles.

Glass melts at 1,400°-1,800° temperatures, depending on the type and colors of the glass. In order to focus the greatest amount of sunlight and reach those temperatures, the lens must be maintained in a perpendicular position realative to the incoming sunlight.

Since the angle of sunlight varies both throughout the day and seasonally, the stand holding the lens must be fully adjustable and able to hold the necessary position. As the sun rises in the morning and lowers in the afternoon the lens needs to tilt vertically, while the lens must also swivel to follow the sun across the sky from East to West.

Want more details? Take a look at the following:

• Sunshine in a Drop of Glass?: How Sundrop Jewelry is made
• Melting Glass with Sunshine: Methods and Techniques

Still more questions? Check out our FAQ.