Water Sphere Lens
Make a lens and a magnifying glass by filling a bowl
with water.
In the 1700s, spherical glass bottles filled with water were used to focus candlelight for
fine work such as lace making. Round bottles of water, left in an open window, have
been known to start fires by focusing sunlight into an intense "hot spot."
A Florence (round-bottomed) flask or transparent spherical bowl, such as a
fishbowl.
A candle or a clear lightbulb with socket.
A white card to use as a viewing screen.
Water.
A sheet of newspaper.
Adult help.
(1 minute or less)
Fill the flask or fishbowl with water to make a water sphere lens.
(15 minutes or more)
Place the light source (the lightbulb or the 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. 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.
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.
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 you use the sun as your light source, the
focused sunlight may be hot enough to burn the paper card. Be careful!
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, and the colors are due
to chromatic aberration.