Femtosecond

A femtosecond is the SI unit of time equal to 10⁻¹⁵ or ¹/_{1,000,000,000,000,000} of a second. That is one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 31.71 million years; a ray of light travels approximately 0.3 µm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a virus.

The word femtosecond is formed by the SI prefix femto and the SI unit second. Its symbol is fs.

A femtosecond is equal to 1000 attoseconds, or 1/1000 picosecond. Because the next higher SI unit is 1000 times larger, times of 10⁻¹⁴ and 10⁻¹³ seconds are typically expressed as tens or hundreds of femtoseconds.

Shorter Times

The waves of visible light oscillate with a period (reciprocal frequency) of about 2 femtoseconds . The precise period depends on the energy of the photons, which determines their color. (See wave-particle duality) This time can be calculated by dividing the wavelength of the light by the speed of light (approximately 3 x 10⁸ m/s) to determine the time required for light to travel that distance.

• 1.3 fs – cycle time for 390 nanometer light, at the transition between violet visible light and ultraviolet
• 2.57 fs – cycle time for 770 nanometer light, at the transition between red visible light and near-infrared