I have a nephew (who is the cutest) so we are exploring many fun and exciting activities we loved as children (and still do today!). We've played with bubbles and hula hoops and someone suggested we start fires with a magnifying glass.... uh no thanks, I'm thinking teaching a young person to start fires is definitely a no-no, especially when they aren't even old enough to hold something steady. This did get me thinking about how long a magnifying glass has been around though!
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 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.
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.
As for starting fires... 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. Let's be careful with this information though, and I'm all for using a magnifying glass for its intended purpose.