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1.1  Light

Light behaves both as an electronmagnetic wave visible to the human eye and as a flux of particles (photons).

1.1.1 Light is an electronmagnetic wave
Monochromatic light is characterised by its wavelength. Multi-chromatic light, which is a
synthesis of several monochromatic lights, is characterised by its spectrum.

Monochromatic light
The human eye only perceives wavelengths lying between 380 (violet color) and 780 (magenta red)  nanometres.

Multi-chromatic light
Multi-chromatic light is a group of several monochomatic lights. It is therefore characterised by its
spectrum which resprents the light intensity of the colors forming it. The eye is incapable of perceiving the spectrum of a monochromatic light; it does not differentiate a monochromatic yellow from a yellow obtained by additive synthesis of a red light and green light.
White light is multi-chromatic; the sunlight spectrum can be separated by a prism.

The white LED light, generated from a blue LED and a yellow phosphor (see different methods of
Combining white light the LEDs in point 6.1), presents an irregular spectrum with the appearance
Pf 2 peaks, strongest in blue (emission of the diode) and weakest in yellow (conversion by blue phosphor into yellow) (figure 3).

Figure 3: Spectrum of a white LED

1.1.2 Light is a flux of particles
All illumination phenomena cannot be explained by the wavy nature of light. The corpuscular nature of light creates light as a flux of ¡°energy particles¡± without mass (photons). It explains the energy related phenomenon associated with light, in particular: fluorescence, phosphorescence, photo-electricity or electroluminescence.
The LED is an electroluminescent light source. When electric current passes, it excites the electrons and moves them to an energy level higher than their equilibrium level. While returning to the normal energy level, the electron releases energy in form of photons.