Images of Fish in Fish Bowls
How do I explain to my child, why do fish look bigger in a
fish bowl? I basically need to explain in child terms, about light
bending when traveling from water to air, and why things are
magnified under water.
Ray optics can be confusing, even to college students. I know. I try to
teach it to them.
You can TRY to explain that the light rays bend convergently when they
travel from the fish through the curved surface of the bowl, making it
look as if they came from a larger fish. But I think that is too
abstract, since you cannot see or touch the path light rays follow; you
can only see the light when it falls on your eye.
I would recommend obtaining a magnifying glass. You can show your child
concretely how looking through the curved glass makes things appear
larger. You could also fill a drinking glass with water and put a spoon
or fork in the water, and show how its size appears to change as you
move it away from the edge of the glass. Your child might then be able
to understand that the same thing happens to the fish.
I remember seeing the Iranian film "The White Balloon," in which the
protagonist, a little girl, is determined to purchase a goldfish for the
Persian new year's celebration, even though her family already has a
goldfish. She wanted the goldfish she saw in the shop because it was
chubby, unlike her family's goldfish. When she finally got the new
fish, it turned out to be no chubbier than the one they already had: it
just looked rounder in the bowl in the shop.
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
I cannot think of any easy words, but I can think of a way to
demonstrate it. You will need a black card with a vertical slit. You
will need a small transparent tray that can hold water. The tray must
have at least one flat side for the light to pass through. You will
need a standard flashlight. Sometimes low power can work as well.
Place the tray on a white piece of paper. Hold, or tape, the card to
the front of the light so that only what passes through the slit will
be seen. Shine the flashlight through the flat side of the tray, from
the air into the water. If held correctly, a line of light should be
seen on the white paper as it passes from air to water. When passing
perpendicular to the tray's side, the light will form a straight line.
As you turn the flashlight so the line enters at an angle, the line will
bend as it passes into the water. This is often seen easiest in a very
dim room. If the light is weak, then a completely dark room might be
After the bending of light is established, drawing the effect on paper
might suffice. Draw a line down the center of the page. On one edge,
draw an eye. On the opposite edge, draw a large fish. You now have the
eye on the side of air and the fish on the side of water. Draw a light
ray with the appropriate bend from the top of the fish to the eye. Do
the same for the bottom. Both will bend inward toward the eye.
Now, cover the side with the fish. Extend the lines on the eye side
over to where the fish is now hidden. They will be further apart than
what they are underneath. Draw a bigger fish to fill the space. You
will now have the smaller fish in the water covered by the larger fish
as it appears. Inform the child that eyes do not see the bend. They
only see straight. The real fish is small (the hidden view). The fish
looks big (the one on top).
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012