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Diver
Find the Fish: How Light Refraction Affects Where To Find The Big One


Lesson Objectives

By the end of this activity, students will:
  1. Relate the successful hunting technique of an eagle to the properties of light.
  2. Be able to explain the behavior of light when changing from one medium to another.
  3. Explain where a fisherman should try to land the bait to have a better chance in catching fish.
Grade Level: Middle school 5-9

National Science Standards

This lesson addresses the following national content standards found in the McRel Standards Database at http://www.mcrel.org/standards-benchmarks/
  1. Knows that an organism's patterns of behavior are related to the nature of that organism's environment
  2. Knows ways in which light interacts with matter (e.g., transmission, including refraction; absorption; scattering, including reflection)
Tools and Materials Needed
  1. Copy of the program "The Living Edens: Tasmania: Land of the Devils"
  2. Rectangular aquarium with straight sides
  3. Liter boxes (available from math supply sources such as EAI Education at 1-800-770-8010 or AIMS Education Foundation at www.aimsedu.org)
  4. Two rulers
  5. Magic markers
  6. Pencils
  7. Unruled paper
  8. Small piece of clay
  9. Wire or pipe cleaner
  10. Protractor
Estimated Time to Complete Lesson
The activity can be completed in one or two class periods, including discussion.

Australasian Gannets Teaching Strategy

Background Information

About 12 minutes into the film, there is a sequence showing a White Breasted Sea Eagle gliding over a river as it fishes. The eagle has become so adept at fishing that it takes almost no time out of the day to obtain its food. This is quite unlike most novice fishermen who spend great quantities of time obtaining few, if any, fish. This activity explores why the eagle has an easier time catching fish.

When the eagle fishes, it flies high above the water looking for a fish to catch. Once a fish is located, the eagle flies to a position where its shadow will not be seen by the fish and closes in on the prey. On the other hand, a novice fisherman tends to randomly toss a baited hook into the water and hope for a bite. Sometimes, if the fisherman is lucky, he/she may be able to spot a fish in the water, especially if polarized-lens glasses are used. Unfortunately, even if the fisherman casts the hook right in front of where he sees the fish, a strike rarely happens.

The reason for the lack of strikes has to do with the behavior of light in water, which can be shown through a simple experiment. When light enters a medium that is more dense, it slows slightly and bends. This is called refraction. The denser the medium, the more the light bends. In the case of the fish, the water is much denser than air and the bending is significant. In addition, the angle to the interface between the mediums where the light strikes the water effects the amount the light bends. The closer that light comes to a 90-degree angle to the surface of the water, the less it bends. At 90 degrees, it does not refract at all.

The fisherman stays on the bank as he casts his line thus giving him a low angle to the water surface. Assuming that the novice sees the fish and perfectly casts the line so it lands right in front of the image, the hook will always overshoot the actual fish. Look at the path of light shown in figure 4 below and assume the fisherman is on the outside of the box. When the light ray strikes the water, it bends though the fisherman can't tell it has been bent. He thinks the fish's position is straight ahead where it would be if there was no bend in the ray.

The eagle starts at a higher angle and has a better idea of the actual location of the fish when it starts to hunt. The position is still a bit off to the eagle, but not as much as what the fisherman sees. As the eagle gets closer to the fish, it glides in at an angle that becomes steeper and steeper until the last second. When it grabs the fish, the eagle is very close to 90 degrees to the surface of the water and it knows just where to grab.

Procedure
  1. Watch the video 'The Living Edens: Tasmania: Land of the Devils' with special attention to the section showing the White Breasted Sea Eagle fishing. The segment is about 12 minutes from the start of the program.
  2. Cover a table with a sheet of newsprint or other large piece of paper. Take an aquarium and place it in the center of the newsprint. Make a small 'fish' out of a piece of pipe cleaner or wire. Using a piece of clay as a base, place the fish in the aquarium so that it sticks up about an inch from the bottom of the tank. It should be positioned so that it is in the middle of the tank, but a distance of about one quarter the length of the long side of the aquarium. (see fig. 1)
  3. Fill the tank with water so that it covers the fish.
  4. Mark the edges of the aquarium by using a permanent marker to draw a line around the entire outside of the aquarium. Using the protractor, look through the side of the tank and draw lines on the paper at 90 degrees on each side. (see fig. 1)
  5. Have students come up to the tank and line up a ruler with the fish. Draw a line along the ruler on the paper pointing at the fish. Students should draw lines from various angles.
  6. Remove the aquarium and post the paper on the board. Take a different colored marker and connect the two 90 degree lines that are across from each other in both directions.
  7. The intersection of these lines is the position of the fish. Use a yardstick to complete the lines that were drawn by the students. None of the student lines will touch the fish.
  8. Give groups of two students a square liter box, several sheets of blank paper and two rulers. Fill the box half way with water and place it in the middle of a sheet of paper in a flat surface. Mark the position of the box with lines.
  9. The students face each other with the liter box in between them and facing opposite sides of the box. Bending down so they look through the water, place the rulers so they appear to line up across the box. Draw lines to show where the rulers were placed. (fig. 2)
  10. Using a second piece of paper, line up the rulers as before but this time have students use as great an angle coming into the box as possible without going around a corner. (fig. 3)
  11. Once finished, remove the boxes and connect the lines as shown in fig. 4 to show the path the light would travel through the water at those angles. Students may wish to try other angles.
  12. Relate the paths of the light through water to what a fisherman on a bank would see and what the eagle would see as it drops down on a fish though statements or written paragraphs.
Helpful Web Sites
Parks and Wildlife Service of Tasmania: Wildlife of Tasmania
http://www.parks.tas.gov.au/wildlife/birds/seaeagle.html
A description of the White Breasted Sea Eagle that contains detail on these birds.

The Reflection and Refraction of Light
http://physics.bu.edu/~duffy/PY106/Reflection.html
Technical information about reflection and refraction of light that includes diagrams and formula

Assessment Recommendations
The aquarium activity should be an ungraded exploration used to generate discussion and interest in the phenomena. The liter box portion may be graded as done or not done. A generalization statement explaining why the eagle rarely misses the fish while the fisherman rarely catches the fish should close the activity. The statement could be made verbally or as a summation paragraph.

Extensions and Adaptations
  1. Students may explore the difference in the refraction of light between fresh water and salt water.
  2. The entire activity may be done as a class demonstration with data taken from the aquarium and examined as a group.
  3. Older students familiar with trigonometry may enjoy calculating the Index of Refraction of the water in the box using direct measurements of similar triangles inside and outside of the box.
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