Michelle Kastner, age 11, of Gig Harbor, Wash., for her question:
HOW DOES A MAGNIFYING GLASS WORK?
A lens is a transparent object that has at least one curved surface. It can be used to form magnified or reduced images, or to concentrate or spread light rays. Most lenses are made of glass, but any transparent material may be used. Lucite or some other plastic, which may be a lot less breakable than glass, can also be used for making most lenses.
A magnifying glass is a lens which makes close objects appear even larger than they actually are. Both sides of the lens are usually curved to form a double convex lens.
You will be able to see two different kinds of images with a magnifying glass. if the glass is held close to a sheet of paper on your desk, you will produce a virtual image. The light rays which produce this image spread out, or diverge, as they pass through the lens and seem to originate on the same side of the lens as the page. The virtual image appears upright and larger than the object.
A real image is seen when rays of light from an object pass through the lens and are focused on the other side. The real image appears inverted, or upside down. The size of the real image depends on the distance of the object from the lens.
The distance from the center of the lens to the point where parallel light rays are focused is called the focal length. If the object is more than twice the focal length away from the lens, the image will be smaller. If the object is less than twice the focal length away, the image will be larger.
The focal length of a lens determines the magnifying power it will have. The greater the curve of a lens, the shorter its focal length. It bends the rays more, and they meet at a smaller distance from the lens.
The focal length of most magnifying glasses is about 10 inches.
Did you know that you can actually start a fire with a magnifying glass? Use great care if you try this experiment, since any flame can be dangerous.
The magnifying glass can be held over a sheet of paper while you are out of doors under the bright sun. Many rays can be focused on a common point on the paper, causing the heat to actually make the paper burn.
Light passing through the center of a thin lens keeps its original direction. But a ray that strikes anywhere else is bent. The amount the light ray is bent increases with its distance from the center of the lens.
Simple lenses cannot form sharp, undistorted, color free images over a wide field. Such defects are corrected by using combinations of lenses.