Water Sphere Lens

Fill the flask or fishbowl with water to make a water-sphere lens.

Place the lightbulb or candle more than 1 foot (30 cm) from the water-sphere lens. Hold the white card against the side of the lens opposite the light source. Caution: Do not use the sun as your light source, because the focused sunlight may be hot enough to burn the paper card.

Move the card away from the sphere until you see an image of the filament (or flame) on the card. Notice that the image is inverted. (Click to enlarge diagram below.)

Move the light source up and notice that its image moves down.

Move the light very close to the water sphere, and notice that you cannot find an image on the card at any distance.

Look through the water-sphere lens at a newspaper held close to the other side of the sphere. Notice that the sphere acts as a magnifying glass. Vary the light-to-lens distance and notice how the image-to-lens distance changes. Also notice how the image changes size.

Light rays from the bulb or candle bend when they enter the water-filled sphere and bend again when they leave the sphere, as shown in the diagram below (click to enlarge). The only light rays that don’t bend are the ones that enter the sphere at a straight-on, 90-degree angle—that is, the ones that pass through the center of the bowl.

The sphere acts just like a lens, focusing the light that passes through into an image on the other side. The image must lie on a straight line from the object through the center of the lens.

The image is upside down and reversed right to left. The motion of the image is also reversed: When the object moves up, the image moves down; when the object moves closer to the sphere, the image moves farther away. It might help to picture a seesaw: When one side moves up, the other side moves down.

Every lens has a focal point, which is a spot where light rays from far away converge after passing through the lens. The distance from the center of the lens to the focal point is called the focal length. Measure the focal length of your lens by finding a bright light source that’s more than 30 feet (9 m) away. Using the white card, find the distance from the lens to the image. This is the focal length.

If an object is closer than one focal length to the center of the water sphere lens, the lens can’t bend the light rays from the object enough to bring them back together to form an image. However, when you look through the water-sphere lens at a nearby object, the lens of your eye can complete the bending, forming an image on your retina. The image on your retina made with the help of the water-sphere lens is larger than the largest image you could make with your eye alone. The water-sphere lens is thus a magnifying glass.

The image you see may be fuzzy and distorted, but should be recognizable. It also may show color distortion. The fuzziness is due to spherical aberration—a sphere is not the perfect shape for a lens; the colors are due to chromatic aberration—each different color of light bends by a different amount as it enters or leaves the lens.

Round bottles of water, left in an open window, have been known to start fires by focusing sunlight into an intense “hot spot.” In the 1700s, spherical glass bottles filled with water were used to focus candlelight for fine work such as lace making. In 2008, Joshua Silver, a former physics professor, designed eyeglasses with fluid-filled plastic lenses. The lenses are each connected to a small syringe so the eyeglass wearer can vary the amount of fluid, changing the power of the lens, until the images are as clear as possible. Silver’s invention is benefiting people around the world who do not have access to eye-care professionals.