A straight line passing through the center of a circle or sphere, connecting two points on its circumference.A diameter is a straight line segment that passes through the center of a circle or sphere, connecting two points on its boundary. It is the longest chord of a circle and exactly twice the length of the radius. In physics, the diameter is a fundamental geometric property used to characterize objects from atomic nuclei to planets, and it appears in formulas for area, volume, and cross-sections, which are essential for calculating scattering rates, gravitational forces, and fluid dynamics.
The concept of diameter dates back to ancient Greek mathematics, with Euclid defining it in his 'Elements' around 300 BCE. The term comes from the Greek 'diametros,' meaning 'measure across.'
For a perfect circle or sphere, the diameter is uniquely defined as the maximum distance between any two points on the surface. In practice, physical objects are rarely perfect, so scientists often use an 'effective diameter'—the diameter of a sphere with equivalent volume or cross-sectional area. This simplification allows complex shapes to be treated with standard geometric equations, making calculations tractable.
Diameter measurements are used in particle physics to determine the size of atomic nuclei, in astronomy to estimate the size of stars and planets, and in engineering to design pipes, lenses, and bearings.
The diameter of a hydrogen atom is about 0.1 nanometers, while the diameter of the observable universe is roughly 93 billion light-years—a ratio of about 10^36.
| θ | variable | angular resolution — angular resolution radian |
| λ | variable | wavelength of light — wavelength metre |
| D | variable | aperture diameter — diameter metre |
| 1.22 | constant | First zero of the Airy disk for a circular aperture — airy disk constant = 1.22 |
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