Light from any point on an object spreads out in all directions. When the spreading light hits the page magnifier lens, it is bent toward the axis of the lens. (The page magnifier is called a positive, or converging, lens because it bends light rays together.)
Page magnifiers have a focal length of about 10 inches (25 cm). A focal length is the distance from the lens to an image the lens makes of a distant object. If an object is farther than one focal length (10 inches [25 cm]) from the lens, the lens can bend all the light that arrives from one point on the object until it comes back to a point on the other side of the lens. This point is a point on the image of the object. If you put white paper at the place where the light rays meet, an image will appear on the paper. An image that can be focused on a piece of paper is called a real image.
However, you don’t need the white paper to see the image. Simply move about 1 foot (30 cm) farther away from the lens than the location of the image, and look at the lens. You will see the image hanging in space. The diagram below (click to enlarge) shows how our eye-brain system follows the light back to the point from which it spreads.
Move your head slightly from side to side and watch the image move. (Actually, your eye-brain system may refuse to interpret the image as hanging in the air. It is so unusual to see something hanging in the air that your brain may insist that the image is on the surface of the lens or even behind the lens—but the image actually is hanging in space.)
If an object is closer to the lens than the focal point, the lens cannot bend the light spreading from the object enough to return it to a point. To your eye-brain system, it looks as if there’s an image on the same side of the lens as the object. This type of image is called a virtual image. A virtual image cannot be focused on a piece of paper (click to enlarge diagram below).
Building an image locator:
You can find the location of a real or virtual image by building an image locator. Push a pin through one end of each soda straw. Use the pins to attach the straws to adjacent corners of the 9 × 9 inch (22 × 22 cm) corrugated cardboard sheet. Push another pin through the other end of one straw to mount it along one edge of the cardboard. The other straw will be free to rotate.
Mount the image locator firmly in place so you can look through the straw fixed to one edge and see one point on the image (click to enlarge diagram below). Then rotate the other straw until you can look through it and see the same point on the image. (You’ll have to move your head to look through the second straw.) The image is located where two imaginary lines, one drawn through each straw, cross. If the image is a real image, you can place a piece of paper there and see it on the paper.
Finding the focal length of your lens:
You can also find the focal length of your particular lens using a bright light source that is more than 30 feet (9 m) away. Caution: Do not use the sun for this!
Hold a piece of paper against the lens on the side opposite the light. Move the paper away from the lens until a sharp image of the light appears on the paper. The distance from the lens to the image is the focal length.