Most magnifying glasses are double-convex lenses and are used to make objects appear larger. This is accomplished by placing the lens close to the object to be viewed. In this way the light rays are bent toward the center of the lens. When these bent rays reach the eye they make the object appear much larger than it actually is. However, if the object is far enough away from the lens, the image will flip, appearing smaller and upside down. The distance at which this flip occurs is twice the focal length (the distance from the optical center of a lens to the point where the light rays converge) of the lens. The focal length of any lens is determined by the amount of curve on the lens’ face. The magnified image is called a virtual image while the smaller, inverted image is called the real image.

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The magnifying glass is one of the most ancient optical devices known to science. Thousands of years ago Egyptians used chips of crystal or obsidian (a type of shiny stone) to better view small objects. In Rome Emperor Nero (a.d. 37-68) was known to have peered through gemstones at actors on a distant stage. The first magnifier constructed for scientific purposes is believed to have been designed by the English philosopher Roger Bacon (circa 1220-1292) sometime during the thirteenth century.

Many people have used a magnifying glass and sunlight to ignite a piece of paper. When the lens is held at exactly two focal lengths from the paper, all of the light will be concentrated into a tiny point, generating enough heat to start a fire.

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The Fresnel lens, sometimes close to 15 feet high, are made designed of concentric rings of glass prisms both above and below; they bend the light into the narrow beam that illuminates the waterways and are powerful enough to cut through the heaviest fog. The beehive design helps to capture and reflect the light and focus the beams out onto the water to help safely guide marine vessels to shore or direct them away from dangerous areas.

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The lenses used in lighthouses are shaped like a magnifying glass as a way to make the concentrated beam even more powerful. When the lenses were first used, fire was the main source of illumination and inventors found that without reflectors being placed behind the flame, it lost nearly 97% of its illumination. The Fresnel lens, because of its unique design, captured all but 20% of its illumination and its magnifying efficiency meant its light beam could be seen more than 20 miles into the darkness.

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Fresnel lenses were found more prevalently in Europe as lighthouse keepers readily accepted the new “technology.” It wasn’t until the late 1800s that lighthouses in the United States were converted because many of the US lighthouse keepers believed the lens was a fad.

Chances are you’ve seen lighthouses and know that they illuminate the waterways along the coastal regions in many areas of the world. You may not have realized though that the lenses that are used in the lighthouses are typically of the Fresnel variety that were invented by a French physicist named Augustin Fresnel in 1822. The lenses have a beehive appearance and the light sits in the center.

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The magnifying glass was the forerunner of the compound microscope (in which a series of lenses are used to focus, magnify, and refocus an image), one of the basic tools used in medicine.