How far does light decay after running? How far can sunlight go?

Obviously, there is an error in your understanding, light has wave-particle duality, that is, it has the characteristics of both particles and waves, light travels in the form of particles, and what spreads out is not an image, it is a real existence of light particles, but your naked eye can't distinguish it, just like taking a car, you sit in the car and leave home, it takes time to reach your destination, can you say that you are an image when you are on the road? It's just that the distance between light and the earth is relatively long, and the distance between you is relatively short, which can only show that the vastness of the universe and the insignificance of human beings are nothing terrible, they are all real things.

As for the attenuation you are talking about, it is related to the medium through which light travels, and it is also related to reflection and refraction.

It has to do with the medium of propagation, and if it's a vacuum, it doesn't decay.

If it's a transparent medium like water, then attenuate by a certain percentage.

Of course, the sun's rays can shine that far, because the universe is a vacuum.

But the light diverges, which means that the farther away you are, the smaller the point of light you see, and there are also phenomena such as redshift, which requires the luminaire to move at high speed relative to the observation point, and if the luminaire is far away from you, then you will see blue light, and if it is close to you, then it will be red light.

And what you mean by attenuation is that the intensity of the light is weakened, right? You feel attenuation because the light is diffused and some photons are absorbed, and the light diffuses because of the interference of the light, or it shines in all directions, such as the sun in all directions, so if it's an ideal parallel light, it doesn't decay in an ideal environment without molecules, otherwise it decays all the time.

The main reason for the attenuation of light is the influence of the medium of propagation.

The process of attenuation is the process of refraction, reflection and other phenomena that occur continuously.

As for whether we see stars from a few years ago or hundreds of thousands of years ago, it can be explained in this way.

According to the wave-particle duality of light, in the process of light propagation, the speed does not change, and the photon will definitely not disappear, then the light will theoretically exist forever, but the energy of the light propagation process will change, and the light energy e=hv (h is Planck's constant; v represents the frequency of the light quantum), the light energy becomes smaller, that is, the frequency of the light wave becomes smaller, and the wavelength becomes longer, which is the so-called redshift phenomenon, and the red light has the longest wavelength in the visible light range.

But the problem is that the wavelength of visible light has a range, and the wavelength of light in the propagation is constantly getting longer, and it will always go beyond the range of naked eye observation, to the wavelength of infrared and radio, maybe special instruments can detect it, but the wavelength can theoretically be infinitely long, until no instrument can detect it, then it is meaningless whether the light exists or does not exist, because even if it exists, we can't detect it, just like dark matter. So dreaming of going beyond the speed of light is to see the desire of childhood largely shattered. Even if you can be twice as fast as the speed of light, it will take you the same time as your age to catch up with your childhood, and fantasy and reality are always very different.

The unit of optical power is dBm, in the instruction manual of the optical transceiver or switch, there is its luminous and receiving optical power, usually the light emitting is less than 0dBm, the minimum optical power that the receiving end can receive is called sensitivity, the unit of the maximum optical power that can be received minus the value of sensitivity is dB(dbm-dbm=db), which is called dynamic range, and the luminous power minus the receiving sensitivity is the allowable optical fiber attenuation value. The value of the actual luminous power minus the actual received optical power during the test is the fiber attenuation (dB).The optimal value of optical power received at the receiver is the maximum optical power that can be received - (dynamic range 2), but this is generally not the case.

Since the dynamic range of each optical transceiver and optical module is different, the specific allowable attenuation of optical fiber depends on the actual situation, and generally speaking, the allowable attenuation is about 15-30dB.

Some manuals will only have two parameters, luminous power and transmission distance, and sometimes it will explain how much the transmission distance is calculated by the attenuation of each kilometer of optical fiber, most of which is divided by the minimum transmission distance, which is the maximum optical power that can be received, if the received optical power is higher than this value, the optical transceiver may be burned out. Divide by the maximum transmission distance, which is the sensitivity, and if the received optical power is lower than this value, the link may fail.

There are two ways to connect optical fibers, one is fixed connection and the other is active connection, fixed connection is fusion splicing, which is to use special equipment to melt the optical fiber to connect two sections of optical fiber together, the advantage is that the attenuation is small, and the disadvantage is that the operation is complex and the flexibility is poor. The active connection is through the connector, usually connected to the pigtail on the ODF, the advantage is that it is simple and flexible, and the disadvantage is that the attenuation is large, generally speaking, the attenuation of an active connection is equivalent to one kilometer of optical fiber. The attenuation of the fiber can be estimated as follows:

Including fixed and active connections, attenuation per kilometer of optical fiber, if the active connection is quite small, this value can be, pure optical fiber does not include active connections, can be reduced to, the theoretical value of pure optical fiber is; For the most part, it is considered good for insurance.

Fiber testing TX and RX must be tested separately, and in the case of a single fiber, only one fiber is used, so of course only one fiber is tested. According to the manufacturer, the implementation principle of single fiber is wavelength division multiplexing, but I think that the use of fiber couplers is more likely.

What is an optical power meter, which is used to measure absolute optical power or the relative loss of optical power through a length of fiber. In a fiber optic system, it's very much like a multimeter in electronics. In fiber optic measurement, optical power meter is a common meter for heavy loads.

By measuring the absolute power of the transmitter or optical network, an optical power meter can evaluate the performance of the optical transceiver. Used in combination with an optical power meter and a stabilized light source, connection loss can be measured, continuity can be verified, and fiber link transmission quality can be evaluated.

Generally, the power meter will let you choose the wavelength, and the difference between different wavelengths is still very large, and it is definitely no problem for tens of kilometers, which can be calculated depending on the acceptance range.

In terms of wavelength, one office is 1310 and 1550

Theoretically it is infinite, as long as the universe is how big it can run far, in a vacuum the light is not attenuated, yes, but the sunlight is scattered around, not concentrated towards one place, the farther away, the less light you will see, the less light will naturally feel less bright, and even think you can't see it, but in fact, that is mainly because the sensitivity of the human eye is not very high, so when the light is less to a certain extent, it is completely dark to the human eye, The reason why many nocturnal animals can move in what people think is completely dark is not because there is really no light in the night, but because the number of lights is so small that the human eye can no longer see it, but because animals can still see this small amount of light. Otherwise, why do you think you can see the Milky Way and other galaxies in places with good weather and little light pollution? That's because the light they emit is not just 12 billion kilometers away from us, it's in terms of light years, wormholes are now just theoretical things in an illusion, and if the light were to travel through it, the scientific community would have been able to prove its existence a long time ago.

Any light source is divergent, and the laser is divergent, and the farther it goes, the more powerful it is. Proxima Centauri, the closest star to our Sun, is light-years away, and I'll give you an intuitive and accurate ratio: if the Sun is a glass marble with a diameter of 1 centimeter, then Proxima Centauri is 290 kilometers away, and its diameter is millimeters (Proxima Centauri is 1 7 times the diameter of the Sun).

This is still the closest star to us, and the emptiness of cosmic space is really unimaginable. It's not that the light can't be transmitted, it's that it's so diffuse that it's almost invisible.

How far can sunlight reach? It has been 100 million years since the birth of the sun, how far has the sun's light traveled in the universe? How far will it spread? What is the farthest distance we can see the sun with the naked eye?

The parameters given by the merchant are all ideal. 3,000 hours of projector lamps are good.

Most of the wavelength bands of light will be strongly attenuated by absorption when propagating underwater, only the blue-green light with a wavelength in the 480 30nm band has the smallest absorption attenuation coefficient in the water, and the strongest penetrating ability, and this band is in the "atmospheric window" of electromagnetic waves

Light waves in the ultraviolet and infrared bands are highly attenuated in water and cannot be used underwater. Blue-green light has the least attenuation, so it is often called this band"Underwater window".

and wavelength light waves with attenuation lengths of 11 m and 2 m., respectivelyThis shows that blue light transmits much better than red light in water.

The propagation of light waves in water.

Among the various waves propagating in water, longitudinal waves have the least attenuation, so sonar technology is widely used. The attenuation of electromagnetic waves is generally so severe that radio waves and microwaves, which are widely used on land, are almost impossible to use underwater. The exception to this is light waves, which are less attenuated than radio waves and microwaves.

This section provides a brief introduction to some of the characteristics of the underwater transmission of light waves.

1.Attenuation characteristics of the propagating beam.

The attenuation law of monochromatic parallel beam propagation in water also approximates the exponential law.

The attenuation coefficient is not only related to the water quality, but also to the wavelength of the propagating beam.

It is customary to use the attenuation length l0 to indicate the magnitude of the attenuation of the underwater propagation beam.

Table 1 Attenuation coefficient of tap water (see doc document).

You don't need to calculate, just take the instrument and measure it to see how much the attenuation is, just see if it reaches the standard.