The magnifying glass is one of the most ancient optical (related to the eye) devices known to science.

By definition a magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle. 

According to encyclopedia.com, 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.

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.

One of the first conceptions of a multi-lens magnification system was by Francastoro of Verona in his Homocentrica of 1535, detailing the use of multiple lenses to further increase magnification properties. By 1590, it is probable that compound microscopes had been developed. Their invention has been credited to Zacharias Janssen. The compound microscope "gradually acquired fashionability” (Ford 19), and a flurry of interest in microscopy followed the publishing of Robert Hooke’s Micrographia in 1665.

Antony van Leeuwenhoek, unknown and untrained in the world of optics, developed a single lens microscope by the late 1600s which had better resolving power than that of compound microscopes. New and interesting specimens placed greater demands on the microscopes of the 1600s, and by the 1700s microscopy was propelled forward by the invention of new optical tools. This is according to Brown University.

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.

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.