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Name: Ashok
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A


Question:
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.



Replies:
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


Ashok,

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 needed.

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
Physics Instructor
Illinois Central College



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