Light is a form of energy that has two aspects:
The color of light is depends on its frequency, which itself depends on the wavelength and the speed of the wave front. The wavelength of an oscillatory phenomenon is usually characterized by the relation:
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Radiation comprising only one wavelength is called monochromatic radiation and radiation which contains several wavelengths is called polychromatic radiation. The collection of all the wavelengths composing polychromatic radiation (and their respective luminous intensities) is called the spectrum.
However, the human eye is not capable of distinguishing the various components of this radiation and perceives only the result, which is a function of the different wavelengths which it comprises and their respective luminous intensity.
The human eye is able to see radiation with wavelengths between 380 and 780 nanometres. The radiation with wavelengths below 380 nm is called ultraviolet radiation, while the radiation with wavelengths above 780 nm tis known as infra-red radiation. The range of wavelengths that are visible to the human eye is called the "visible spectrum" :
Thanks to the cornea (the translucent envelope of the eye) and the iris (which by closing allows the amount of light entering the eye to be regulated), an image is formed on the retina. The latter is made up of rods and cones.
The rods, which contain a pigment called rhodopsine and are located in periphery of the retina, make it possible to perceive luminosity and movement (scotopic vision), while the cones, located in a region called the fovea, make it possible to differentiate the colors (photopic vision). There are actually three kinds of cones:
In addition, it should be noted that the sensitivity of the human eye to luminous intensities related to the three primary colors is not the same:
There are two types of color synthesis:
When light falls on an object, some wavelengths are subtracted since they are absorbed by the object. What we see is the combination of the wavelengths that are reflected or transmitted (i.e. those that are not absorbed). This process is used in photography and for the impression of colors. The secondary colors are blue, the red and the green: