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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.[1] For context, a femtosecond is to a second as a second is to about 31.7 million years; a ray of light travels approximately 0.3 µm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a virus.[2]

The word femtosecond is formed by the SI prefix femto and the SI unit second. Its symbol is fs.[3]

A femtosecond is equal to 1000000 zeptosecond, 1000 attoseconds, or 1/1000 picosecond, 1/1000000 nanosecond. 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.[4]
• The waves of visible light oscillate with a period (reciprocal frequency) of about 2 femtoseconds ${\lambda\over{c}} = {(600\times10^{-9})\over{(3\times10^8)}} = 2.0\times10^{-15}$. 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.[5]
• 1.3 fs – cycle time for 390 nanometer light, at the transition between violet visible light and ultraviolet[5]
• 2.57 fs – cycle time for 770 nanometer light, at the transition between red visible light and near-infrared[5]
• 200 fs – the swiftest chemical reactions, such as the reaction of pigments in an eye to light[5]
• 300 fs – the duration of a vibration of the atoms in an iodine molecule[6]