< h1 class="pgc-h-arrow-right" data-track="1" > review</h1>
After learning some basic physics, we know that the fastest moving object in the universe is light, and we also know that the dazzling starlight we can see on the earth may actually be those stars emitted tens of thousands of years ago, after a long period of propagation to the sky above the earth, the same reason, when we wave the flashlight on the earth to the sky, these lights will also enter the cosmic space, to the places we do not know, so how do they spread?
<h1 class="pgc-h-arrow-right" data-track="3" >What is light? </h1>
Before we understand the propagation of light, let's first recall the definition of light. Before contact with physics, ordinary people basically do not notice the existence of light, although our life scene constantly appears in this substance, it can be seen, but can not be touched, open the flashlight will appear, turn off the flashlight will disappear, to call it a substance always feels less appropriate.
What exactly is light? This problem has a long history, as early as two thousand years ago, the ancients began to observe and study it, the first breakthrough in the characteristics of light came from our sage Mozi, who conducted experiments on small hole imaging very concisely proved that light travels in a straight line.
Westerners came to this conclusion more than 200 years later than Mozi. In that era of backward scientific experimental thinking, although there was already a consciousness of studying light, the scope of research could only be limited to some optical phenomena, and some simple laws could be summarized from them, and the question of the nature of light was not well promoted for a long time.
In the backward era of experimental science, what light is, like what man is and what the universe is, is full of mystery, and philosophers have proposed various explanatory models to try to depict the truth of the composition of matter. The Roman Lucretius was the first to propose particle theory, believing that light is composed of countless tiny particles, and he applied Democritus's theory of the atom, arguing that light is actually no different from most matter, and that it is particles in its most basic composition.
In the following thousands of years, this problem and the controversy associated with it have continued, but due to the lack of empirical means, no substantial progress has been made, and in the early 20th century, experimental science has made rapid progress, various instruments and research methods have emerged in an endless stream, and physicists have slowly unlocked the true face of light in the laboratory.
Light is actually the result of the apparent flow of photons, the so-called photon flow is actually the movement of electrons after obtaining a specific energy, and this movement has a specific frequency, more than a certain frequency, the electron will move in the form of light, below a certain frequency, it will show other forms.
The activity of electrons switching between different frequency bands because of the difference in energy size is called transition, such as when our flashlight is turned on, that is, the electron is stimulated by external energy, and then forms a specific frequency of motion in a short period of time, and releases high-energy photons in the process, which are manifested as bright beams in the visual imaging of the naked eye, and other luminous objects we see in daily life are actually no light themselves, the reason why they can be seen by us, It is because the photons reflect the sun.
<h1 class="pgc-h-arrow-right" data-track="11" > propagation of light</h1>
Therefore, photons are real matter, although we turn off the switch, but it will not disappear, but will continue to move in space, although it will change due to different energy, but it will not disappear completely as we intuitively see.
So what exactly does light look like when it travels? First of all, Mozi's small hole imaging and many subsequent scientific studies have verified one of its most important features, that is, propagation along a straight line. Looking back at our lives, we have never seen the light that will bend, the sun's rays are the same, and because of this, we can see our own shadows on the ground on a sunny day, if it is a curved light, then the transmitted portrait must be mutilated.
Secondly, light travels in the form of waves, and some people may find it strange, isn't it just said that it is propagated along a straight line? How can it now be said that it propagates in the form of waves? In fact, these are two different concepts, the propagation of light along a straight line is the trajectory of the entire photon stream, and the propagation of it in the form of waves is to say that all the electrons that make up this photon stream have a specific frequency when they propagate in space, and this frequency determines their wavelength, and the wavelength determines their color.
After the rain, the floating water droplets in the air decompose the sunlight like a prism, and the photons of different wavelengths are emitted to different positions, so there is a colorful rainbow we see, and when the weather is good, you can always see the pleasant blue sky and the result of the scattering of the blue light in the sunlight by the earth's atmosphere.
<h1 class="pgc-h-arrow-right" data-track="16" > conclusion</h1>
A beam of light seems simple, but this is only the conclusion that the naked eye tells us, under the more sophisticated instruments, they have a more complex internal composition, for humans, it seems to lose its meaning after the completion of illumination, but the light itself will not disappear, before encountering obstacles, it will continue to travel through space.