Friends of Steemians, this time I bring you a very interesting subject like LIGHT.

The primitive idea about vision was based on the theory that the eye projected a stream of particles that made objects visible, later the theory spread that the same objects emitted particles and penetrated the eye. It was believed that the stream of particles moved in a straight line, since it was impossible to see behind the corners. But both theories had to be rejected.

When the water wave passes through a narrow opening, on the other side of it, new waves are generated, the light waves behave in the same way.

In the year 1801, Thomas Young demonstrated with an experiment that, given the characteristics of light, it should be described as a transverse wave, but the origin of the waves remained a mystery.

Later Heinrich Hertz managed to produce the first radio waves. On the other hand, the English J. C. Maxwell had foreseen the existence of such waves and had calculated their speed. It was already known that the speed of light was the same as that of radio waves (300,000 km / sec).

<center>http://astronomy.nmsu.edu/geas/lectures/lecture17/pics/lec17_em.png
[Source](http://astronomy.nmsu.edu/geas/lectures/lecture17/slide01.html)</center>
Obviously, light waves and radio waves had the same type of wave movement constituted by magnetic and electric fields.

Light can be influenced by matter. The electric field of radiations varies continuously due to the electrons in the atoms of matter.

Dispersion, refraction and reflection are processes that can cause changes in the direction and character of light waves.

This light transformation information helps us to give us detailed visual impressions of the world around us. On the contrary, in the vacuum of space the light moves without changes, the light of the stars that a space traveler can contemplate is invariable.

#### Dispersión
Due to molecules, dust or irregularities in the air layers, part of the atmospheric light is scattered, especially blue light. Due to this phenomenon, during the day the sky is blue; during the sunset, the sky acquires a red color, this due to direct light.
<center>http://images.tutorvista.com/content/human-eye-colourful-world/white-light-dispersion.jpeg
[Source](https://www.tutorvista.com/content/science/science-ii/human-eye-colourful-world/dispersion-white-light-prism.php)</center>
#### Refraction
When light passes from air to a denser matter, like water, it changes direction and refracts.
The refraction of light is not the same for all wavelengths (colors), therefore, water droplets can divide sunlight into different colors, forming the rainbow.
<center>http://www.physicsclassroom.com/Class/refrn/u14l4b3.gif
[Source](http://www.physicsclassroom.com/class/refrn/Lesson-4/Rainbow-Formation)</center>

#### Reflection
When light strikes any surface, it is reflected, and part of it is retained, absorbing its energy.
Generally the reflection surface is not smooth and rejects light in many directions. The surface of the water may or may not reflect light according to its state.

<center>https://www.desktopclass.com/wp-content/uploads/2010/11/Mirror348.jpg
[Source](https://www.desktopclass.com/notes/physics/reflection-of-light-10th-physics-lesson-14-1-2.html)</center>

## What is the Light?
Light is defined as an electromagnetic wave that is composed of small particles called photons and that allows us to visualize everything that surrounds us, providing color and sense to the eye.
<center>https://4.bp.blogspot.com/-wU7s548admU/TZtvj0JyRPI/AAAAAAAAC3E/6C6in3QS0Ak/s1600/Raydan_230_60_mini-4.jpg
[Source](http://cs-fs-primero.blogspot.com/2013/03/clasificacion-de-fuentes-de-luz.html)</center>
### How is the Light produced?
Atoms and molecules can accumulate thermal energy and store it for a short time, in a state of excitement, in the excited atom, the excess energy is emitted in the form of electromagnetic radiation, whose intensity and wavelength depend on the amount of Energy. Available energy In an incandescent object, many atoms emit radiation whose wavelength is within the limits of the visible spectrum.

### Light producers
The sun and fire give light and heat at the same time, circumstances that man has known how to take advantage of since he learned to use fire.
<center>https://image.freepik.com/vector-gratis/radial-de-fondo-la-luz-del-sol_293-194.jpg
[Source](https://www.freepik.es/vector-gratis/radial-de-fondo-la-luz-del-sol_686684.htm)</center>
The atoms of an element move faster as the temperature increases, when they collide they can collect kinetic energy and store it, so that they can be excited, their internal structure changes when this excitation occurs and they do not return to stabilize until it falls off.
The extra energy received, Part of this energy is released in the form of electromagnetic radiation, if the energy stored has been little, infrared radiation or heat radiation, whose wavelength is long, if it has been much, shortwave ultraviolet radiation.
In a small area of ​​energy between the two, the radiation acquires the wavelengths or frequencies that the eye registers and interprets as LIGHT.

### Sources of light
All objects continuously emit electromagnetic radiation. From about 600 ° C, part of the radiation acquires a frequency so high that it can be observed in the form of light by the eye. As the character of the radiations depends on the temperature, they are called temperature radiations.

All the other forms of light are due to luminescence. In the fluorescent tube, the kinetic energy of an electron beam is transformed into light. In the case of the firefly, the light is produced as a result of a chemical process.

#### Bioluminescence
Many animals have their own light, because of a chemical process that takes place in their luminous organs where excited atoms are produced whose energy allows them to emit light. The chemical energy has transmuted into light, without atom collisions, and without heat production.

#### Firefly
<center>https://www.animales.website/wp-content/uploads/2016/08/La-luci%C3%A9rnaga.jpg
[Source](https://www.animales.website/la-luciernaga/)</center>

#### Deep-water fish
<center>http://3.bp.blogspot.com/-ddm8huBxJrA/TdYUUldoGpI/AAAAAAAAEKU/h6NQ6nn2lZY/s400/pez%2Babisal%2Babismal%2Bbv.jpg
[Source](http://100curiosidadesdelmundo.blogspot.com/2011/05/los-peces-abisales-y-bioluminiscentes.html)</center>


#### Phosphorescence
The phosphorescence is due to the same causes of fluorescence, the light emitted always has a longer wavelength than the absorbed, however, the phosphorescent material, unlike the fluorescent, can emit a slight glow, even several hours after having been excited.
<center>https://upload.wikimedia.org/wikipedia/commons/thumb/0/09/Tritium-watch.jpg/440px-Tritium-watch.jpg
[Source](https://es.wikipedia.org/wiki/Radioluminiscencia)</center>

#### Luminescence
It is the property of emitting light without raising the temperature. In the case of the TV, the light material inside the screen and the gas in the neon tube are excited by electron beams.


Sources of references:
[Source](https://en.wikipedia.org/wiki/Light) 
[Source](https://www.tutorvista.com/content/science/science-ii/human-eye-colourful-world/dispersion-white-light-prism.php)