Speed of light in s i system is 3 into 10 power 8 metre per second what is the second of light in a new system of units in which unit of length is x kilometre and unit of why my second
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The speed of light in vacuum, commonly denoted c, is a universal physical constantimportant in many areas of physics. Its exact value is 299,792,458 metres per second(approximately 300,000 km/s (186,000 mi/s)[Note 3]). It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1/299792458 second.[Note 4][3] According tospecial relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universecan travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum (including electromagnetic radiationand gravitational waves). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalenceE = mc2.[4]
Speed of light
Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth.
Exact valuesmetres per second299792458Planck length perPlanck time
(i.e., Planck units)1Approximate values (to three significant digits)kilometres per hour1080000000miles per second186000miles per hour[1]671000000astronomical units per day173[Note 1]parsecs per year0.307[Note 2]Approximate light signal travel timesDistanceTimeone foot1.0 nsone metre3.3 nsfrom geostationary orbit to Earth119 msthe length of Earth'sequator134 msfrom Moon to Earth1.3 sfrom Sun to Earth (1AU)8.3 minone light year1.0 yearone parsec3.26 yearsfrom nearest star to Sun (1.3 pc)4.2 yearsfrom the nearest galaxy (the Canis Major Dwarf Galaxy) to Earth25000 yearsacross the Milky Way100000 yearsfrom the Andromeda Galaxy to Earth2.5 million yearsfrom Earth to the edge of the observable universe46.5 billion years
The speed at which light propagates throughtransparent materials, such as glass or air, is less than c; similarly, the speed ofelectromagnetic waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels atc / 1.5 ≈ 200,000 km/s (124,000 mi/s); therefractive index of air for visible light is about 1.0003, so the speed of light in air is about 299,700 km/s (186,220 mi/s), which is about 90 km/s (56 mi/s) slower than c.
For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. The speed of light can be used with time of flight measurements to measure large distances to high precision.
Ole Rømer first demonstrated in 1676 that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism.[5] In 1905, Albert Einsteinpostulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source.[6] He explored the consequences of that postulate by deriving the theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism.
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (983571056 ft/s; 186282.397 mi/s) with ameasurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in theInternational System of Units (SI) as the distance travelled by light in vacuum in 1/299792458
Speed of light
Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth.
Exact valuesmetres per second299792458Planck length perPlanck time
(i.e., Planck units)1Approximate values (to three significant digits)kilometres per hour1080000000miles per second186000miles per hour[1]671000000astronomical units per day173[Note 1]parsecs per year0.307[Note 2]Approximate light signal travel timesDistanceTimeone foot1.0 nsone metre3.3 nsfrom geostationary orbit to Earth119 msthe length of Earth'sequator134 msfrom Moon to Earth1.3 sfrom Sun to Earth (1AU)8.3 minone light year1.0 yearone parsec3.26 yearsfrom nearest star to Sun (1.3 pc)4.2 yearsfrom the nearest galaxy (the Canis Major Dwarf Galaxy) to Earth25000 yearsacross the Milky Way100000 yearsfrom the Andromeda Galaxy to Earth2.5 million yearsfrom Earth to the edge of the observable universe46.5 billion years
The speed at which light propagates throughtransparent materials, such as glass or air, is less than c; similarly, the speed ofelectromagnetic waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels atc / 1.5 ≈ 200,000 km/s (124,000 mi/s); therefractive index of air for visible light is about 1.0003, so the speed of light in air is about 299,700 km/s (186,220 mi/s), which is about 90 km/s (56 mi/s) slower than c.
For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. The speed of light can be used with time of flight measurements to measure large distances to high precision.
Ole Rømer first demonstrated in 1676 that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism.[5] In 1905, Albert Einsteinpostulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source.[6] He explored the consequences of that postulate by deriving the theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism.
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299792458 m/s (983571056 ft/s; 186282.397 mi/s) with ameasurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in theInternational System of Units (SI) as the distance travelled by light in vacuum in 1/299792458
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