0.1 The Golden Ratio 0.1.1 Finding the Golden Ratio In this unit, we will continue to study the relation between art and geometry. During the renaissance period, many artist, architects, and sculptors used ratios of distance in their work. An example of one of these special ratios is called the Golden ratio. The Golden ratio can be found by dividing a segment into two pieces where the ratio of the longer piece to the shorter is the same as the ratio entire line segment to the longer piece. (See illustration below.) a b = a+b a a = Shorter Piece of the line segment b = Longer Piece of the line segment a + b = Entire line segment In the next passage, we will find the value of the Golden ratio by solving above proportion for a. a b = a+b a a = b(a + b) a = ab + b a ab b = 0 Now, use the quadratic formula to solve for a a = b+ b +4b a = b+ 5b a = b+b 5 Now, substitute back into the original ratio to get the Golden ratio. ϕ = a b = b+b 5 b = b+b 5 1 b = b(1+ 5) 1 b = 1+ 5 = 1.61803... It turns out that the Golden ratio is an irrational number meaning that the number does 1
not repeat or terminate. Recall that the number π pi is also an irrational number. The Golden ratio was referred to as the divina proportione meaning divine proportion by Luca Parcioli in 1509. The famous physicist Johann Kelper later called it the sectio divina in 1610 which translates to divine section. The term Golden ratio or Golden section came into use around 1640. Many artists and composers have since used the Golden ratio in their work including famous artist Leonardo da Vinci and composer Bela Barlok. 0.1. The Parthenon and the Golden Rectangle The Golden rectangle is a special rectangle where the ratio between the length and the width is the Golden ratio ϕ 1.6 1 = 1.6 = ϕ Typically, the value 1.6 is used for the Golden ratio to make computations easier. The Greeks believe that Golden rectangles where more pleasing to the eye than other rectangles. The Golden Rectangle can be found in the measurement in many structures of ancient Greece. In Athens, buildings such as the Parthenon on the Arcopolis contains several Golden rectangles. Using the picture of the parthenon above, we a can draw a rectangle around the perimeter of the parthenon as shown in the next diagram. After making this rectangle, we will measure the length and width of the rectangle.
Now, let s find the ratio between the length and the width of the rectangle drawn over the perimeter of the Parthenon. This value will be very close to the Golden ratio. LengthofRectangle idthofrectangle = 7.5 4.6 = 1.6304 The next diagram illustrates how several Golden rectangles can be found in the Parthenon. You will later discover that this pattern of Golden rectangles is very similar to the Nautilus Spiral. 0.1.3 The Golden Cross A Golden cross is a cross that is constructed using two special ratios. e start by defining length of the upper portion of the cross as T and the length of the lower portion as B. e will also define the overall height of the cross as T+B and the width of the cross as. (See illustration below:) 3
A cross with the measurements shown above is a Golden cross if the following ratios equal the Golden ratio. B T B+T = ϕ = 1.6, B B+T = ϕ = 1.6, and = ϕ = 1.6 In the next example, we check to see if the cross is a Golden cross. Example 1 Is the following cross shown below a Golden cross? Solution: Check all three ratios to see they are equal to the Golden ratio. B T = 4.86cm 3cm = 1.6 B+T B B+T = 7.86cm 4.85cm = 1.6 = 7.86cm 4.85cm = 1.6 4
Example Find the values of T, H, and that will make the cross a Golden cross. (Round your answer to the nearest hundredth.) Solution: First find the value of T by setting the following ratio equal to the Golden ratio. 8 = 1.6 T T ( 8 ) = 1.6(T ) T 8 = 1.6T T = 8 1.6 T = 4.9in Now, find H by using the fact that H = T + B. Therefore, H = 8 + 4.9 = 1.9 inches Now, set the following ratio equal to the Golden ratio (1.6) to find. 1.9 = 1.6 ( 1.9) = 1.6( ) 1.9 = 1.6 = 1.9 1.6 = 8.0in Example 3 Given the value of the top segment of a cross is T = 10 in Find the value of H and that will make the cross a Golden cross. (Round your answer to the nearest hundredth.) Solution: First find the value of B by setting the following ratio equal to the Golden ratio. B 10 = 1.6 10( B 10 ) = 1.6(10) B = 16. in. 5
Now, find H by using the fact that H = T + B. Therefore, H = 10 + 16.7 = 6.7 inches Now, set the following ratio equal to the Golden ratio (1.6) to find. 6.7 = 1.6 ( 6.7) = 1.6( ) 6.7 = 1.6 = 6.7 1.6 = 16.5 inches 0.1.4 The Golden Box The Golden box is a rectangle solid whose length (L), width (), and height (H) satisfy the Golden ratio. These Golden Ratios are define as: = L = ϕ = 1.6 H Example 4 Find the width and length to the nearest tenth of a Golden box that has a height of 3 centimeters. (Round your answer to the nearest hundredth.) Solution: First, find the value of the width by setting up a ratio using the Golden ratio. 6
= 1.6 3in 3in( ) = 1.6(3in) 3 = 4.86 in Now, find the length of the box. L = 1.6 4.9in 4.9in( L ) = 1.6(4.9in) 4.9in L = 7.9in 0.1.5 Art and the Golden Ratio The Golden ratio can be found in the artwork of many famous artist. In particular, the Golden ratio can be in some of Leonardo Da Vinci works. The next picture is a photo of Leonardo self portrait. In the next picture, it is shown how a Golden rectangle can be constructed around Leonardo face in his self portrait. Notice that the width of the rectangle is.49 cm and the height of the rectangle of 4.0 cm. 7
If you divide the height of the rectangle by the width of the rectangle, you will get the Golden ratio. Heigth idth = 4.0.49 = 1.614 Other Examples of objects that use or contain the Golden Ratio or Golden Rectangles Another classic example of an object that contains the Golden ratio or the Golden rectangle is the Nautilus Spiral. The Nautilus is a seashell that can be divided into several golden rectangles. The illustration below shows how the Nautilus Spiral can be divided into at least 8 golden rectangles. 8
0.1.6 Exercises 1. Using a straightedge or a ruler, try to draw a Golden rectangle around the Mona Lisa s face. Confirm your results by dividing the longer side by the shorter side.. Find the width and length to the nearest tenth of a Golden box that has a height of 8 centimeters. 3. Find the height and length to the nearest tenth of a Golden box that has a height of 6 centimeters. 4. Find the width and length to the nearest tenth of a Golden box that has a height of 1 centimeters. 5. Find the value of T, H, and that will make the cross a Golden cross. 9
6. Find the value of T, H, and that will make the cross a Golden cross. 10