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The speed of light, usually denoted by c, is a physical constant representing the speed at which light and all otherelectromagnetic radiation travels in vacuum. Its value is exactly
299,792,458 metres per second[
1][2] (approximately 186,282 miles per second)
. In the theory of relativity, c connects space and time, and appears in the famous equation of mass--energy equivalence
E = mc2.[3] The speed of light is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the speed of gravity and of gravitational waves and an upper bound on the speed at which energy, matter, and information can travel.
The speed at which light propagates through transparent materials, such as glass or air, is less 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 at c / 1.5 ≈
200,
000 km/s; therefractive index of air for visible light is about 1.0003, so the speed of light in air is very close to c.
Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of
Jupiter's moon Io. After centuries of increasingly precise measurements, in
1975 the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion. In
1983, the metre was redefined in the
International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[1][2]
A light-year is a unit of distance. It is the distance that light can travel in one year.
Light moves at a vA light year is a way of measuring distance. That doesn't make much sense because "light year" contains the word "year," which is normally a unit of time. Even so, light years measure distance. You are used to measuring distances in either inches/feet/miles or centimeters/meters/kilometers, depending on where you live. You know how long a foot or a meter is -- you are comfortable with these units because you use them every day. Same thing with miles and kilometers -- these are nice, human increments of distance. When astronomers use their telescopes to look atstars, things are different. The distances are gigantic. For example, the closest star to
Earth (besides our sun) is something like 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. That's the closest star. There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn't a practical unit to use because the numbers get too big. No one wants to write or talk about numbers that have 20 digits in them!
So to measure really long distances, people use a unit called alight year. Light travels at 186,000 miles per second (
300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light year is the distance that light can travel in a year, or: 186,000 miles/second * 60 seconds/minute *
60 minutes/hour * 24 hours/day *
365 days/year = 5,865,696,000,000 miles/year
A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way!
Using a light year as a distance measurement has another advantage -- it helps you determine age.
Let's say that a star is 1 million light years away. The light from that star has traveled at the speed of light to reach us. Therefore, it has taken the star's light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, our sun is 8 or so light minutes away
. If the sun were to suddenly explode right now, we wouldn't know about it for eight minutes because that is how long it would take for the light of the explosion to get here.
How Far is a Light-Year?
A light-year is a unit of length used by astronomers to measure interstellar distance (the distance between stars). A light-year is defined as the distance that light will travel in a year. If the speed of light is 186,000 miles per second (300,000 km per second), then calculate the distance that light will travel in one year.
Express your answer in miles per year. [
Note: Cancel similar units above and below the dividing line. This process is known as "dimensional analysis."]
Solution:
186,000 mi/sec x 60 sec/min x 60 min/hr x 24 hr/day x 365 days/yr
- published: 30 Jul 2010
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