Thanks to the Light we can see the world around us. Everyday images such as: a tree or mountains, also small things like bacteria or very distant objects like stars and galaxies, all in various colors but ...

Light is electromagnetic radiation. When inside an atom, an electron passes from a high level of energy to a lower one, it emits a packet of energy which we call photon . "Photo" means light in Greek. Hence come words like Photography or Photosynthesis. 

The photon goes off at almost 300 thousand kilometers per second and creates an electric and magnetic field that moves in the form of waves. Photons that carry a lot of energy do so at high frequencies, such as gamma rays, with wavelengths that are less than an atom. Photons that carry less energy have lengths of several meters or even kilometers, such as radio waves. This whole wavelength variety is called the "electromagnetic spectrum". 

And it happens that human eyes evolved to see a small part of him: from violet to red. How can we see? As sure you know white light is composed of radiation of all colors. When an object receives light, depending on the substance it is made of, it absorbs some wavelengths and reflects others. Those that it reflects are what we see as its color. An object to see, absorbs all wavelengths less those of the green spectrum, a white object reflects almost all and black absorbs almost all.

The retina is like a mosaic of cells where photoreceptors are found: cones and rods, which are neurons that are sensitive to light. 

When the photons reach the cones and rods a chemical reaction is triggered that generates a nervous impulse that reaches the brain where it is interpreted. Most human beings have three kinds of cones, each sensitive to a wavelength or color, that's why we say we have trichromatic vision. Other animals such as dogs only have two kinds of cones, bees have four kinds of cones.