In the field of atomic and molecular physics, an "atomic unit of length" is a unit of measurement that is used to express distances at the atomic and molecular scale in a dimensionless way. It is part of a system of atomic units (a.u.) that simplifies calculations involving fundamental physical constants and properties of atoms and molecules.
The atomic unit of length (a.u. of length) is defined in terms of the Bohr radius (a₀), which is a fundamental constant in atomic physics. The Bohr radius is approximately 0.52917721067 angstroms (Å) or 5.2917721067 x 10^-11 meters (m).
In atomic units, the Bohr radius is set to exactly 1 a.u. of length. Therefore, when using atomic units, distances are expressed relative to the Bohr radius, and the value of 1 a.u. of length corresponds to the typical size scale of atomic and molecular structures.
The use of atomic units simplifies many quantum mechanical calculations and allows physicists and chemists to work with dimensionless quantities, making it easier to compare and analyze atomic and molecular properties.
A "pole" is a unit of length that is equivalent to 16.5 feet or 5.0292 meters. It is sometimes referred to as a "rod" or a "perch," and these terms are often used interchangeably. Poles, like rods and perches, were historically used in some English-speaking countries, primarily in the United Kingdom and the United States, for land measurement, surveying, and real estate.
While the use of poles, rods, and perches has become less common in modern times, you may still encounter these units of length in certain contexts, especially in older land descriptions or surveys. In everyday measurements and official standards, more commonly used units like meters or feet are preferred.