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What is light?

Light is a form of energy that has two aspects:

• An electromagnetic wave aspect
• A corpuscular aspect (photons)

The concept of colour

The colour 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:

λ = CT

• λ represents the wavelength
• C represents the speed of the wave fronts
• T indicates the period of the wave (in seconds)

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"  :

It is possible to separate the spectral colours using a crystal prism.

The operation of the human eye

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 colours (photopic vision). There are actually three kinds of cones:

• Those mainly sensitive to red radiation (570 nm), called erythrolabes
• Those mainly sensitive to green radiation (535 nm), called chlorolabes
• Those mainly sensitive to blue radiation (445 nm), called cyanolabes

• Protanopes, who are highly insensitive to red
• Deuteranopes who are highly insensitive to green
• Trinatopes who are highly insensitive to blue

In addition, it should be noted that the sensitivity of the human eye to luminous intensities related to the three primary colours is not the same:

Additive and subtractive synthesis

There are two types of colour synthesis:

• Additive synthesis results from the addition of light components. The light components are directly added to the emission; this is the case of monitors or colour televisions. When the three components, red, green, blue (RGB) are added, white is obtained. The absence of components yields black. The secondary colours are cyan, magenta and yellow because:
  • Green combined with blue yields cyan
  • Blue combined with red yields magenta
  • Green combined with red yields yellow

  

• Subtractive synthesis makes it possible to restore a colour by subtraction, starting from a white light source, with filters for the complementary colours: yellow, magenta, and cyan. The addition of filters for all three colours yields black and their absence produces white.

  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 colours. The secondary colours are blue, the red and the green:

  • Magenta (primary colour) combined with cyan (primary colour) yields blue
  • Magenta (primary colour) combined with yellow (primary colour) yields red
  • Cyan (primary colour) combined with yellow (primary colour) yields green