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 "pica" is a unit of measurement commonly used in typography and desktop publishing to represent the size of text, fonts, and other elements on a printed page. The pica is part of a system of measurement used in the field of typesetting, which involves arranging and designing text for printed materials.
In the pica system:
1 pica (abbreviated as "p") is equal to 12 points. 1 point (abbreviated as "pt") is approximately equal to 1/72nd of an inch.
So, 1 pica is roughly equal to 1/6th of an inch.
This system is based on the traditional point system used in printing, where a point is a very small unit of measurement. Picas and points are still widely used in the design and printing industry, especially when specifying font sizes, line spacing, and other layout details for printed materials like books, magazines, and newspapers.
In digital design and modern desktop publishing software, you can often switch between using picas and points or other units like inches, millimeters, or pixels to set and adjust the layout of text and graphic elements.