A large hanging lens creates upside-down images of distant objects and right-side-up images of nearby objects. You can locate the upside-down images by using a piece of white paper as a screen. The right-side-up images are harder to find.
CAUTION: Do not use the sun as a light source for this activity. It will burn paper and damage eyes.
- Large plastic page-magnifier Fresnel lens, 6 × 9 inches (15 × 22 centimeters) or larger (be sure you don’t get a wide-angle viewer lens: to test this, hold your hand an inch or so behind the lens and look through the plastic; your hand should appear larger, not smaller, than normal size)
- Two or three large binder clips
- Objects to look at and a lamp or computer screen
- Large sheet of white paper
- Two soda straws
- Common pins
- Corrugated cardboard or foamcore sheet, 9 × 9 inches (22 × 22 cm)
- A partner
Stand a few feet back from the lens and look through it at objects on the other side. Notice that distant objects appear upside down; nearby objects will appear right side up.
Stand close to the lens. Hold your hand close to the lens on the other side. You will notice that your hand is magnified and right side up.
Stand an arm’s length from one side of the lens; have a partner stand an arm’s length from the other side. Look at your partner's face through the lens. Have your partner move closer to the lens as you back away, keeping the same two-arm’s-length distance between the two of you. Then reverse this procedure: You step closer to the lens while your partner moves away. Notice how your partner's face appears, and ask how your face appears.
Find an object that is brightly illuminated (such as a light bulb or computer screen), and dim the lights in the rest of the room. Hold the lens at least several feet from the object. Hold a large piece of white paper against the side of the lens, away from the object. Holding the lens still, slowly move the paper away from the lens until an image of the object comes into focus (click to enlarge diagram below).
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.
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.
The Fresnel lens used in the photo above is made out of wedges of plastic. The wedges must be thicker at the edge in order to bend light more, and thinner in the center (click to enlarge diagram below). Run your finger over the ridges of the Fresnel lens and notice that the ridges are higher near the edge and lower and smoother near the center.
The Fresnel lens is named for August Fresnel, who figured out how to make these lenses for the French lighthouse commission in the late 1800s. Lighthouses needed large lenses to gather the light from a lamp and make it into a beam. If such a lens were ground out of glass, it would be thick, heavy, and expensive. Fresnel realized that the bending of light at the lens occurred at its curved surface, and that the thick glass had a minimal role in image formation. He figured out a way to maintain the curvature of the surface while getting rid of the useless glass. He made his lighthouse lenses out of prisms.