What is a light year

What is a light year


  • 1 Light
  • 2 light year
  • 3 Examples of the use of light year
  • 4 measuring the speed of light 4.1 Galileo attempt 4.2 method of Romer 4.3 speed of light in vacuum
  • 4.1 Galileo attempt
  • 4.2 method of Romer
  • 4.3 speed of light in vacuum
  • 5 References

the light

Long before the nineteenth century was the concept that a flood of light particles that are either exported from the eye, either from the body that look at him. The idea that a light particles emanating from objects that we see this has led the world's idea of ​​Isaac Newton, and used this idea to explain the phenomena of reflection and refraction. [1]

Acceptance remained the scientists to the idea of ​​Newton until 1678 where he suggested physicist and Dutch astronomer Christiaan Hoegns (Baghanngelezah: Christian Huygens) that the light is a kind of waves, and was able to wave theory of interpretation Huggins phenomena of reflection and refraction of light. In 1801 managed the world Thomas Young (in English: Thomas Young) prove that the light wave, by making light interferes, which will lead to a decline in the intensity of light (or the disappearance of the entire light) or increase the intensity of light (or light intensity doubled) The these phenomena Tarafan destructive interference and overlapping construction respectively. Then the right of publication of Maxwell (in English: Maxwell) for his work in electricity and magnetism in 1873, which also supported the wave theory of light. [1]

Managed to wave theory of light of the interpretation of most of the phenomena of optical, but they failed to explain some phenomena such as the phenomenon of PV (in English: Photoelectric Effect), the phenomenon through which we see the start of an electron from the metal surface when you shed light on it, and the wave theory of light failure is that the kinetic energy of each electron does not depend on the severity of the incident light, but on the frequency, while the number of electrons emitted from the surface of the metal depends on the intensity of light falling on the metal. Famous scientist Albert Einstein was able to (in English: Albert Einstein) of this interpretation of the phenomenon in 1905 drawing on the concept of muzzle energy the world's Max Planck, and as a result of interpretation of this phenomenon and his Nobel Prize in Physics in 1921. [1] [2]

Keep us now answer the question of what light, is it a wave, or a particle? The answer is not as simple as we have already seen, but obviously experimentally (the fact that physics knowledge depends on experience) shows that the light behavior in the exact wavelength sometimes, and in some cases, other special objects appear behavior. [1]

Light year

Light-year is a unit of distance used within the scope of the galaxy, the label may seem misleading some thing because of the use of the word "year" will think it is the unit of time. In case we wanted to talk about greater distances from the galaxy can use the other distance which is astronomical farsakh. [3]

One light-year is equal to the distance light travels in one (365.25 days a year) in a vacuum. In the words of simpler, the one light year is equal 9,460,730,472,580.8 km, or 9.46 × 1015 m approx. [3]

Examples of the use of light year

Astronomers usually uses a light-year or farsakh astronomer within the Milky Way; As the astronomer farsakh approximately equal to 3.3 light years, for example:

  • The distance between us and the Super Nova Cancer (English: Crab Supernova) is 4000 light years.
  • Extend our galaxy - the Milky Way - on the length of almost 150,000 light years.
  • Away from us Andromeda galaxy - the closest galaxy to the Milky Way - a distance of 2.3 million light years. In our solar system, we use a different unit for light year, an astronomical unit (in English: Astronomical Unit) as the astronomical unit one is equal to approximately 150 million kilometers, the distance between the sun and the earth, which is also equal to 1.58 × 10-5 light years (ie 8 minutes of light), and the distance between the sun and Mercury are almost one-third of the distance between the Earth and the sun, which is 2.66 minutes of light. But outside the solar system. [4]

Measuring the speed of light

Throughout history there have been many attempts to measure the speed of light, and like everything else logical thinking speed law is equal to the distance divided by time, but with the light, it's not as simple as it is very fast, and in the following are some attempts to measure the speed of light.

Galileo attempt

World Galileo Galilei asked (in English: Galileo Galilei) of two of his stand on the two mountains separated by a distance of 10 km and gave each one of them a lantern covered, if opened, one lantern, then, the other open the bottle when the light seen from the first lantern, and record time between the disclosure of the lantern The first disclosure of the lantern which will be the second time required until the light travels the distance between the two mountains. But it was instantaneous, and was not able to record time, which led to the realization of the impossibility of Galileo measure the speed of light in this way. [1]

Way Romer

In 1675 the astronomer Danish Ole Romer (in English: Ole Roemer) first successful attempt to measure the speed of light using the observations of cosmic moon of Jupiter Io (in English: Io), as Ayew needs 42.5 hours for until the complete cycle around Jupiter, while the cycle buyer need around the sun for twelve years ground, which means that when moving the amount of land angle distance of 90 degrees, the buyer will have cut angle distance of 7.5 degrees. [1]

Must be Ayew fixed cycle, and change the time required to complete this course means that the moon is either slowing down or it is accelerating, if it slows down it will fall to the buyer, and if it accelerates, it will get away from him and become free in space, but none of this did not happen , which means that Ayew has a fixed and regular fixed time cycle. [1]

After a year during which Romer collects regular observations of the moon Io, noted that there is a difference in the timing of the Ayew course on average! The observations indicate that the cycle Ayew need longer time when the earth will be away from the buyer, while the session will be shorter when the earth is approaching from the buyer. But since the Ayew has a fixed course he should have been Romer sees eclipse of the moon Io when it enters the area under the buyer (ie, when no is received light from the sun even reflected because it has become behind the buyer, which means that it will not be received no light from the sun), and including that its fixed should be able to predict the next Balkhosov to Ayew, but he was not able to do so, as the eclipse was delayed schedule when the land away from the buyer, even if we take the period between Rsidin to eclipse the moon to be 3 months, the time delay in the eclipse will be 600 seconds. . This delay because of the distance between the Earth and Io has changed it in the first monitoring reported in the second monitoring. [1]

Using observations Romer enables Hoegns to estimate the speed of light, as it is as much as the amount of the speed of light to be 2.3 × 108 m / s is very important, as it proved that limited the speed of light (ie, it is not infinite). [1]

The speed of light in vacuum

After that rolled experiments and attempts to measure the speed of light, and it was Physio first by the speed of light by a somewhat low error relative to the possibilities that were available in his time and found that the speed of light is 3.1 × 108 m / s, and later became the results more accurate, so we got to an error ratio of the amount of 1 m / s (a very low error rate as the speed of light is 299 792 458 m / s. [5]


  • ^ أ ب ت ث ج ح خ د ذ Raymond A. Serway, John W. Jewett (2004), Physics for Scientists and Engineers , United States : Thomson Brooks/Cole, Page 1095, 1096, 1097. Edited.
  • ↑ "The Nobel Prize in Physics 1921", Nobel Prize, Retrieved 23-12-2017. Edited.
  • ^ أ ب "Measuring The Universe", International Astronomical Union , Retrieved 23-12-2017. Edited.
  • ↑ "What is a light-year and how is it used?", Star Child Question, Retrieved 16-1-2018. Edited.
  • ↑ Philip Gibbs (1997), "How is the speed of light measured?"، Internet Archive, Retrieved 23-12-2017. Edited.


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