Overview



|Overview | |

|Lines, circles, squares, triangles. These geometrical objects belong to an ancient system of geometry | |

|which dates back over 2000 years to the Greek mathematician Euclid. Euclidian geometry is an effective | |

|tool for describing and measuring most of the shapes of objects in the world. However, a relatively | |

|new system of geometry, called fractal geometry, describes objects found in the natural world with more| |

|complex shapes. These shapes seemingly defy geometrical categorization. The shapes of coastlines, | |

|the branching of trees or the leaf patterns of ferns are examples of fractal structures. Fractal | |

|geometry enables researchers to analyze the shapes of things in nature, and compare one system to | |

|another. So it helps us define structure in new and useful ways. Secondly, if an object in a geological| |

|or biological context has fractal geometry, it informs how that system was formed or grew. So the | |

|fractal analysis provides clues about how things in the world are made. Fractals are fun and once you| |

|learn the rules for making a fractal, you can make up your own! | |

Background

This new system is called fractal geometry and can be easily described in its basic form to middle and high school students. One of the hallmarks of a fractal structure is that it shows “self-similarity”. Self similarity simply means that the object has geometrical features that reappear at different scales. Simply put, as you change scale, or zoom in, at the pattern of the fractal object looks the same at different scales. Fractal geometry has helped us categorize and quantify the shapes of objects that were previously too complex to evaluate. Of course, the subject of fractal geometry can get quite sophisticated and involve very advanced mathematics. However, some mathematical manipulations that help students understand fractals are quite simple. Furthermore, simply drawing or constructing models is perhaps the best way to explore the idea of self-similarity. Fractals provide a wonderful way to introduce students to a different way of thinking about math and geometry.

Materials

For each student:

• Student sheets

• Pencils/pens

• Unlined paper

• Graph paper

• Ruler

• Construction paper

• Scissors.

Engage

Show students several visual examples of both natural and algorithmically (computer) generated fractals. You may use web resources to find these examples and there are links at the end of this chapter that may be helpful. Some examples of natural fractals are ferns, branching trees, snowflakes, river paths, coast lines, etc. Have students brainstorm ideas about what is similar about these objects. What shapes do they have? Can we use our familiar shape categories (squares, triangles, cubes, etc.) to define them?

For more advanced students introduce the idea of self-similarity as explained above. For younger students explain that sometimes patterns or shapes occur within an object at different scales. A fern is a typical example of a natural fractal. Its overall frond shape is repeated at smaller and smaller scales. Natural fractals have a limit to how many levels of self-similarity they can have (ferns have perhaps 3 or 4 levels). In contrast, mathematical fractals, have infinite levels of self-similarity. Contrast fractal-like objects with non-fractal objects such as simple geometrical shape like an individual solid square or circle. For example a car (the wheel on a car is not “car shaped”) and a dog (the paw is not “dog shaped”).

A simple Fractal: The letter L.

The best way to gain understanding of fractals is to simply look at an example.

Building fractals is a great way to introduce the concept of “algorithms”. An algorithm is simply a recipe or a series of instructions. In the case of fractals, the algorithm is quite simple. You repeat a single instruction several times and a fractal is generated. The instruction is simply to take an object of a certain shape, then make a new bigger object of the same shape, using the original object as a building block. For example here is an algorithm to make a fractal out of the letter L.

1. Draw (or create on a computer) a letter L .

2. Now make an “L” out of copies of your original letter L. We will use 5 copies of the original shape (but you could simply use three).

This is the crucial step in the algorithm for the students to fully understand. The new object is in the same general shape as the original object. The specific rule we used here was to apply a linear magnification of three to our original object (we used three L’s down and three across). In a linear magnification sense, our new “L” is 3 times bigger than our original. We will repeat this magnification in each step.

3. Now we perform another iteration and make a larger L out of the previous structure. Again three down and three across (5 copies of the structure from step 2):

4. Repeat again. Take the whole structure you made in step three and make an L with 5 copies of it. Three down and three across. Things begin to look interesting!!

Use an example like the one above to introduce the idea of generating a fractal. Once they have the idea of an iterative algorithm (repeated instruction), they will be ready to move onto Student Sheet 1. It is good to emphasize both the math and the beauty of the fractal. It looks cool! But imbedded in the structure is a mathematical design. By looking at the fractal carefully, you can determine the rules that generated it.

Explore

Dimensionality. Before going further, we will review some geometrical concepts. In particular, we need to review the concept of dimensionality. Objects are usually described as having one, two or three dimensions. Although all objects are truly three dimensional, we can describe a flat piece of paper as being two dimensional or a string or wire as being one dimensional. In the case of the paper and string, we ignore the finite thickness of the object and pay attention only to those dimensions which are most prominent (the length and width of the paper and the length of the wire). The dimensionality of the object describes how it fills space. More precisely, the dimension of the object describes how it fills space as it is scaled to larger or smaller sizes. We will also introduce another term: linear magnification. By linear magnification, we mean scaling an object in size such that any dimension of the object (length or width in the case of the piece of paper), increases by the linear magnification. So if we have a piece of paper and scale it by a linear magnification of two, its length is increased by two and its width is increased by two. We’ve simply increased the object size by two. However, the new area of the object is four times the original area. So for a simple two dimensional object such as a piece of paper, a linear magnification of two, increases the amount of two dimensional space that is filled by the object by a factor of four. The same space-filling idea can be applied to a three dimensional object. If we increase the diameter of a solid sphere by a factor of two, we increase the amount of three dimensional space filled by that object by a factor of eight (two cubed). So conventional (non-fractal) one, two and three dimensional objects fill space in a particular and familiar way. As we shall see, fractals fill space in a very different way.

A Mathematical Fractal: The Sierpinski Triangle

The Sierpinski triangle was discovered by polish mathematician Waclaw Sierpinski (1882-1969) in the early 20th century and is what we would now call a fractal. Sierpinski was interested in “self-similar” sets of shapes. Self-similarity means that two objects share the same overall shape. As we go from iteration to iteration in the following exercise, each resulting shape has the same overall outline, and is therefore self-similar to the iteration that preceded it (and all other iterations). A normal solid triangle can be simply enlarged to make a self-similar large triangle, but the properties of the larger triangle are pretty much the same. Both the small and large solid triangle cover the same fraction of the area within their borders (all of it: 100%). What’s very curious about the Sierpinski triangle is that as we go through the iterations making self-similar structures, it covers a smaller and smaller proportion of the area within its overall borders (3/4, then 9/16, then 27/64….). This property of the object means that it has a fractal dimension between 1 and 2. It is neither 2 dimensional nor 1 dimensional but somewhere in between.

Give each student Sheet 1 and ask them to create the Sierpinski triangle. This activity can be done a number of ways. They can simply draw the triangles with pen and paper or they can draw them on a computer (all of the fractals in this chapter were made by the authors with the drawing tools in Microsoft word… copy, paste, repeat…) The fractals can also be make using construction paper and scissors. Have the students generate dozens or hundreds of small equilateral triangles and build the fractal up from there. How many levels can they achieve?

Student Sheet 1 provides the answers to the first few iterations of the Sierpinski triangle. The answers to interations 4-8 are below. Iterations 7 and 8 may be well beyond their capability but they may make for a nice longer term group project where triangles are added over time to a big fractal on a chalk board or poster board.

Iteration 1. Draw an equilateral triangle:

Iteration 2. Make two copies and touch vertices to make a larger triangle:

Iteration 3. Make two copies of this whole structure and arrange as you did in step 2.

Iteration 4. Repeat

Iteration 5: Repeat

Iteration 6: Repeat (not to scale)

Iteration 7 and 8: Repeat (not to scale)

.

Many resources on the web describe this fractal in detail.

• For example:

Explain

Explain how fractal geometry is a new way of describing shapes of objects that occur in nature. Just like the line, square, and circle, are mathematical tools that help us describe and evaluate the shapes of certain objects in our world, fractals help us with the more complex shapes around us such as ferns, trees, mountain ranges, etc. They help us categorize and quantify shapes that seem to defy easy categorization.

Fractal Dimension

One of the fascinating properties of fractals is that they fill space in different ways than more familiar non-fractal objects. We are used to categorizing geometrical objects as having 1, 2 or 3 dimensions. As mentioned above, fractals can have non-integer dimension. To determine the fractal dimension of an object, we perform the same sort of iterations we did in the previous exercises and determine how the number of copies we need per iteration and the size (linear magnification) of the object scale relative to one another.

The fractal dimension (D) is related to the number of pieces (N) and the magnification (m) in the following way

N = mD

The fractal dimension D is then

Log(N) = Log (mD)

Log(N) = D Log (m)

D = log(N)/log(m)

Example 1: Non-Fractal Objects: Lets do an example for which we know the answer and see how the equations work. A solid square is 2 dimensional.

N = mD Where m=2 and N=4. We need to solve for D which is the dimension.

We take the log of both sides

log(N)=log(mD) The properties of logarithms allow us to pull the parameter D out in front.

log(N)=D log (m)

Solving for D

D = log(N)/log(m) = log(4)/log(2)

= log (22)/log (2)

= 2*log(2)/log(2)

= 2

The square is 2 dimensional

Example 2: Fractal Objects: Now repeat the exercise with a fractal object and see how the equations determine fractal dimension. The first two Sierpinski triangle stages are shown below.

Let’s now look at going from the 2nd to 4th iteration in the Sierpinski triangle. This should give the same answer if the object has a well defined fractal dimension (which it does).

Another very interesting potential topic of discussion is why certain natural objects have fractal geometry. Students may have some interesting insights or theories on this which will certainly be informed by their experience with this investigation.

Extend

For a more challenging addition to this activity, have students explore fractal dimensions found on Student Sheet 2. This investigation will introduce the idea of objects with dimension between 1, 2, and 3 integer dimensions.

Sections 1, 2 and 3 on Student Sheet 2 introduce students to a method of determining the dimension of a geometrical object based on concept of self similarity. They will gain confidence in this method by verifying that a line, square and cube, are one, two and three dimensional respectively. They will then move on to determining the fractal dimension of the Sierpinski Triangle in section 4.

In section 5, the students are challenged to determine the dimensionality of what is known as the Koch Curve while in section 6 the students are challenged to try to make a fractal with dimension between 0 and 1.

Evaluate

Ask students express in their own words the difference between something that is a fractal and something that is not. Have students define “self-similarity” in their own words. Invite students to create their own fractal and determine how well it fits the definition of fractal geometry. For more advanced students, challenge them to evaluate the dimension of their own fractals or to set out to create a fractal that has dimension within a certain range (1-1.5 vs. 1.5-2).

Most importantly, this exercise is intended to instill in students a sense that mathematics is interesting and imaginative. Fractals provide a student the opportunity to go on their own personal adventure exploring the intersection of mathematics and science through the production of their own unique creation.

References:







An extension of this concept is the Pythagorean Tree. The web has a number of excellent resources (search for “Pythagorean Tree”). One site has a generator program for various versions of this fractal:



Student Sheet 1: Building the Sierpinski Triangle

The Sierpinski Triangle is a special mathematical set discovered by Waclaw Sierpinski in the early 1900s.

Iteration 1. Draw an equilateral triangle:

Iteration 2. Make two copies and touch vertices to make a larger triangle:

Iteration 3. Make two copies of this whole structure and arrange as you did in step 2.

Iteration 4: Repeat

Iteration 5: Repeat

Iteration 6: Repeat (This will be challenging!!!)

Student Sheet 2: Fractal Dimension

One of the strange properties of fractals is that they can have dimension different from 1 2 or 3. In fact, they can have “non-integer” dimension meaning their dimension can be somewhere between 0 and 1, between 1 and 2, or between 2 and 3. This investigation challenges you to calculate the fractal dimension of different objects.

To determine the dimension of an object, we perform the same sort of steps we used in the previous exercises. For each step, we look at how many copies of the original piece we need to make the next bigger one. We also look at how much bigger the new piece is. By relating the number copies of the original we need (N=number of pieces) and how much bigger the new piece is (m=magnification), we can determine the “fractal dimension”, D through the following equation.

N = mD

The fractal dimension D is then

Log(N) = Log (mD)

Log(N) = D Log (m)

D = log(N)/log(m)

Let’s look at some familiar objects first to see how the equations work and to make sure they give us the answer we expect.

1. A line

Let’s look first at a line, which we know is one dimensional. If the If we magnify the line by a factor of 2, we need to simply place two of the original pieces side by side. This object is “self similar to the first” meaning it has the same shape.

So the magnification is 2 (m=2) the number of pieces we need is 2. Use the formula

D = Log(2)/Log(2) = 1

The line has a dimension of 1. Its one dimensional! Of course!

2. A square

Let’s now look at a familiar 2 dimensional object: a square. If we magnify by a factor of 2 we need 4 pieces. When we magnify by 2, we mean that we “blow up” the object so that the length any side is twice as long. It’s like changing the magnification on a microscope or a telescope by a factor or 2. The resulting object must have the same shape, or in other words, the new object is “self similar” to the original: it is a square, just larger. So the magnification is 2 (m=2) and the number of pieces required is 4 (N=4).

D = Log (4)/Log(2) = 2 Log(2)/Log(2) = 2

The square is 2 dimensional.

Note that we could also magnify by 3 and get the same answer. Magnifying by 3 means the length of the side of the new square is 3 times wider. If the square is 3 times longer on each side, we need 9 squares to make the new square.

So m = 3 and N = 9. D = Log(9)/Log(3) = 2 Log(3)/Log(3) = 2. It is still 2 dimensional.

3. A cube

Lets try a three dimensional object like a cube. Magnify by a factor of 2. Again this means the new bigger cube is twice as long along each side. To make a cube twice as long along each side, we need 8 cubes.

So m = 2 and N = 8 (we need 8 cubes to make the bigger cube).

D = Log (8)/Log(2) = Log(23)/Log(2) = 3 Log(2)/Log(2) = 3

It is 3 dimensional!!

4. Non-integer Dimension: The Sierpinski Triangle

5. Challenge:

What is the fractal dimension of the following fractal?

6. Challenge:

Can you make a fractal that has fractal dimension between zero and 1?

Example: Here is one possible answer, although there are many

. The following fractal is called Cantor’s Dust.

Iteration 1:

Iteration 2:

Iteration 3:

Iteration 4 (scaled)

Iteration 5 (scaled)

The dimension of this fractal:

Each iteration magnifies by 3 and requires 2 copies

D = Log(2)/Log(3) = 0.631.

[pic]

-----------------------

In going from iteration 1 to iteration 2 we are magnifying by a factor of 2 (each side of the triangle is twice as long). So m=2. We need 3 triangles to do so. So N=3.

D = Log(3)/Log(2)

D = 1.585

The Sierpinski triangle has dimension 1.58. It is between 1 and 2 dimensions!!

Objectives

• Investigate the properties of fractals

• Define fractal geometry

• Develop skills in creating fractals

• Calculate the fractal dimension of an object

Process Skills

• Observing

• Inferring

• Predicting

• Analyzing data

Activity Duration

1 hour

Check: Let’s go from the iteration 2 triangle to the iteration 4 triangle. We’ve magnified by a factor of 4 and we need 9 copies of the iteration 2 triangle to make the iteration 4 triangle. So m=4 and N =9

D = Log(9)/Log(4)

D = Log (32)/ Log (22) = 2 Log (3)/ 2 Log(2) = Log(3)/Log(2)

D = 1.585

Again it works out to D= 1.585.

Answer:

As we go from iteration 1 to iteration 2, we need 4 pieces (N=4) and we’ve magnified by a factor of 3 (m =3). This is readily apparent by observing that the object is 3 times longer and taller in the next iteration.

D = Log(4)/Log(3) = 1.262

[pic] [pic][pic]

Natural Fractals in nature. Left: Fern (actually computer generated). Middle: Tree . Right Bacterial culture.

Pictures must be redrawn.

Tree

Fern:

Petri Dish Bacteria:

D= Fractal dimension

N= number of pieces

m= magnification

D= Fractal dimension

N= number of pieces

m= magnification

Magnify by 2

Magnifying by 2 means the width or height of the new object is twice as large

The new object must have the same shape, or must be “self-similar” to the original. It should still be square. So we need 4 copies of the original square to make the new larger square. We’ve magnified by 2 (m=2) and we need 4 square to do it (N=4)

2

4

1

3

We are magnifying by 2. Magnifying by 2 means the width or height of the new object is twice as large

Calculation:

N = mD m=2 ; N=3

log(N)=log(mD)

log(N)=Dlog(m)

D=log(N)/log(m)

D=log(3)/log(2)

D=1.585

m=2

We need 9 copies of the smaller triangle to make the new larger triangle. We’ve magnified by 4 (m=4) and we need 9 copies to do it (N=9)

D= log(9)/log(4) = 1.585

We get the same answer.

N=4

We are magnifying by 4. Magnifying by 4 means the width or height of the new object is 4 times as large

The new object must have the same overall shape, or must be “self-similar” to the original. So we need 3 copies of the original triangle to make the new larger triangle. We’ve magnified by 2 (m=2) and we need 3 triangles to do it (N=3)

[pic]

The Sierpinski Triangle. In each step, a new triangle is made from 3 copies of the previous structure.

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