Symmetry can be found all around us. You need only look in the mirror to find evidence that the human body is symmetrical. If you draw a line down the middle of your body, you will see that one side is the mirror image of the other side: two eyes, two ears, two arms, two legs, ten fingers, etc.
The type of symmetry first mentioned in the video segment is called bilateral symmetry, which is also called reflection symmetry or mirror symmetry. A figure has bilateral symmetry if you can draw a vertical or horizontal line through the middle of the figure and one half of the figure is a mirror image of the other half. The line down the middle is called a line of symmetry. In the video segment, the first leaf shown by the plant expert exhibits bilateral symmetry.
Another common type of symmetry is rotational symmetry. If a figure can be rotated a certain number of degrees about its center and look exactly the same, the figure is said to have rotational symmetry. The angle through which the figure can be rotated to make it look the same is called the angle of rotation. For example, the five-pointed star can be rotated 72 degrees about its center, and it will look exactly as it did in its original position. An equilateral triangle, which has sides of equal length, can be rotated 120 degrees about its center and it will still look the same. Many objects in nature, such as some flowers and snowflakes, exhibit rotational symmetry. In the video segment, the flowers that exhibit rotational symmetry all had five petals, which means that their angle of rotation would be 360/5 degrees, or 72 degrees.
Plants and animals that generally exhibit symmetrical features are thought to be healthier than asymmetrical members of their species. Scientists believe that humans and animals inherently associate symmetrical features with a strong immune system, which is seen as a likely predictor of strong and healthy offspring. Researchers have found that animals and humans often seek out mates with symmetrical features.
In addition to nature, symmetry exists in many man-made objects and is an important concept in art, science, and architecture. When choosing their brand logos, companies have taken advantage of the fact that the human eye is attracted to symmetry. If you think about some of the sports team and company logos you know, you may recognize that many of them exhibit bilateral or rotational symmetry.
To learn about the symmetry found in snowflakes, check out Snowflake Physics Flash Interactive.