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What is a light-year


Before the 19th century, the concept of light was a flow of particles emanating from the eye or object we are looking at. Regarding the idea that light is particles that emanate from the objects we see, scientist Isaac Newton led this idea and used this idea to explain the phenomena of reflection and refraction.

The scientific acceptance of Newton’s idea remained until 1678 when the Dutch physicist and astronomer Christian Huygens proposed that light was a type of wave and Huygens’ wave theory was able to explain the phenomena of reflection and refraction of light. In 1801, Thomas Young was able to prove that light is a wave, causing interference of light, which would result in a decrease in the intensity of the light (or the complete disappearance of the light) or an increase in the intensity of the light. the light. (or a doubling of light intensity) These two phenomena are called destructive interference and constructive interference respectively. Later, Maxwell published his work on electricity and magnetism in 1873, which also supported the theory of light waves.

The lightwave theory has been able to explain most of the optical phenomena, but it has failed to explain some phenomena such as the photoelectric effect, the phenomenon through which we see the release of an electron from the surface of the metal by projecting light. about it, and the failure of the theory of light waves lies in the fact that the kinetic energy of each electron does not depend on the intensity of the incident light, but rather on its frequency, while the number of electrons emitted by the metal surface depends on the intensity of the light incident on this metal. The famous scientist Albert Einstein (English: Albert Einstein) was able to explain this phenomenon in 1905 using the concept of energy quantization developed by Max Planck, and as a result of his interpretation of this phenomenon, he won the Nobel Prize for a physicist in 1921.

Now we have the answer to the question of what light is, is it a wave or a particle? The answer is not as simple as we have seen previously, but it is experimentally clear (the fact that physics is a science that depends on experience) that light sometimes exhibits wave behavior and at other times it exhibits specific behavior of bodies.


A light-year is a unit of distance used in the range of the galaxy. The name can be misleading due to the use of the word “year” and is believed to be a unit of time. If we want to talk about greater distances in the galaxy, another unit of distance can be used, which is the astronomical parsec.

A light-year is a distance that light travels in a year (365.25 days) in a vacuum. In simpler terms, one light-year equals 9,460,730,472,580.8 km, or approximately 9.46 x 1015 meters.

Examples of using a light year

Astronomers often use the unit of light-year or parsec in the Milky Way. Since the astronomical parsec equals about 3.3 light-years, for example:

  • The distance between us and the Crab Supernova is 4,000 light-years.
  • Our galaxy – the Milky Way – is roughly 150,000 light-years long.
  • The Andromeda Galaxy, the closest galaxy to the Milky Way, is 2.3 million light-years away. As for our solar system, we use a different unit of a light-year, which is the astronomical unit, because an astronomical unit equals about 150 million kilometers, which is the distance between the sun and the earth, which is also equal to 1.58 x 10-5 light-years (or 8 light minutes), and the distance between the sun and Mercury is about one-third of the distance between the earth and the sun, or 2.66 light minutes. But outside the solar system.

Measurement of the speed of light

Throughout history, there have been many attempts to measure the speed of light, and like everything else, it makes sense to think that the law of speed equals distance divided by time, but with light, it’s not that simple, because it is very fast, and here are some attempts to measure the speed of light.

Galileo attempt

The scientist Galileo Galilei (English: Galileo Galilei) asked two of his assistants to stand on two mountains 10 km away and gave each a covered lantern. The first is revealing the second lantern, which will be the time it takes for the light to cross the distance between the two mountains. But it was instantaneous and unable to record time, which led Galileo to realize that it was impossible to measure the speed of light in this way.

Romer’s method

In 1675, the Danish astronomer Ole Roemer made the first successful attempt to measure the speed of light using astronomical observations of Jupiter’s moon Io (English: Io), because Io takes 42.5 hours to complete one full rotation around Jupiter, while a cycle of Jupiter around the Sun takes twelve Earth years, which means that when the Earth moves at a 90-degree angle, Jupiter will have traveled an angular distance of 7.5 degrees.

Ayu must have a constant cycle, and the change in the time it takes to complete this cycle means that the moon slows down or speeds up, and if it slows down it will fall on Jupiter, and if it speeds up, it will escape from him and be free. in space, but none of this has happened, which means that Io has a constant time constant and a uniform cycle.

After a year of Rumer collecting regular observations of Io’s moon, he noticed there was a difference between Io’s cycle time and the mean! Observations have indicated that the Io cycle needs a longer time when the Earth is farther from Jupiter, while the cycle is shorter when the Earth approaches Jupiter. But since Io has a fixed cycle, Rumer should have seen a lunar eclipse, Io when it enters the shadow region of Jupiter (i.e. when sunlight does not reach it until it reflects it because it has become behind Jupiter, which means that no sunlight will reach it), its rotation is constant. He should have been able to predict Io’s next eclipse, but he couldn’t, because the eclipse was delayed when the Earth was far from Jupiter. If we take the period between two observations of a 3-month lunar eclipse, the eclipse delay time will be 600 seconds. This delay is because the distance between the Earth and Io changed in the first observation of the latter in the second observation.

Using Romer’s observations, Huygens was able to estimate the speed of light, as he estimated the speed of light to be 2.3 x 108 m / s, which is very important because he showed that the speed of light is finite (ie it is not infinite).

The speed of light in a vacuum

Subsequently, many experiments and attempts were made to measure the speed of light and Physio was the first to calculate the speed of light with a rather low error rate compared to the capabilities available at its time and found that the speed of light is 3, 1 x 108 m / s, after which the results became more accurate until an error rate of 1 m / s was reached (which is a very small error rate because the speed of light is 299,792,458 m / s.

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