A femtosecond is the SI unit of time equal to 10−15 of a second. That is one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second what a second is to about 31.7 million years.
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.
- Typical time steps for molecular dynamics simulations are on the order of 1 fs.
- 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.
- 200 fs – the swiftest chemical reactions, such as the reaction of pigments in an eye to light
- 300 fs – the duration of a vibration of the atoms in an iodine molecule
See also 
- "Femtosecond: Merriam Webster definition".
- NIST. "[[NIST]] Definitions of the SI units". Wikilink embedded in URL title (help)
- "Femtosecond: use in molecular dynamics simulation".
- Andrew M. Weiner (2009). Ultrafast Optics. Wiley. ISBN 978-0-471-41539-8.
- Abbi, S. C. (2001). Nonlinear Optics and Laser Spectroscopy. United States of America: Alpha Science Int'l Ltd. p. 361. ISBN 8173193541.