Circles are special ellipses that have a single constant radius around a center. Circles and their various properties, such as the radius, diameter, circumference and area, have applications in real life. If the radius of the circle is known, all the other parameters can be calculated. Important information can be determined for real life applications based on the parameters. For example, knowing the circumference of the planets helps us compare their relative sizes, and artificial satellites that are launched travel in orbits of certain circumference. The radius of curvature of a camera lens can be used to determine its focal length, which is the distance from the lens where light rays will focus. Pizza and cake sizes are determined by the diameter of the pan in which they are baked, and the distance travelled by a person sitting on a Ferris wheel can be determined by calculating the circumference of the wheel. Tires of different vehicles can have different radii. The circumference of a ring needs to match the circumference of the finger on which it is worn for a good fit.
