"Well, that was where the mistake was, there was no field. It was just that when you shook one charge, another would shake later. ....Shake this one, that one shakes later. The Sun electrons shakes; my eye electron shakes eight minutes later, because of a direct interaction across.” (Richard P. Feynman, Nobel Lecture, December 11, 1965).

So, that is light: a long distance interaction between moving electrical charges. But charge movement must occur with a non zero acceleration ("...shake..."), that is with a variable speed. For example, electrons moving within the antenna of your mobile telephone will make electrons in different conducting bodies to move in the same way and transmit a set of coded signals in the form of radio waves. The difference between light and and radio waves consist in wavelength only. Whereas radio wave may have a wavelength of about 1 cm, visible light spans over a wavelength region from about 200 to 750 mn.

In conclusion, radiation is energy traveling in the form of particles or waves in bundles of energy called photons. It can apply a variable force on any charged particle and impart it a variable acceleration. As the acceleration is inversly proportional to mass, lighter particles (such as electrons) will be particularly disturbed. Electrons bounded i atoms/molecules are obliged to accept only specific energy level (energy quantization) and light absorbtion or emision by such systems must obey the equation E₁ - E₂ = hf, where E₁ - E₂  is the energy difference between involved levels, f is wave frequency and h is Planck's constant. Conversely, no restriction applies to free electrons (such as electrons in plasma).

For longtime, the experimenter eye was the light receptor in spectroscopy experiments. Retina contains a light receptor called Rhodopsin. It consists of  a large protein to whom Retinal  is bound. Light absorption turns the red double bond in Retinal into a single bond and this allows 11-cis-retinal to turn into into 11-trans-retinal. This conformational change in Rhodopsin activates an associate  protein and triggers a second messenger cascade and finally the brain receives a signal. The "shaken" electron, in this case, is located in the pi-bonds system of retinal and must obey the energy quantization rule.
 

A rhodopsin  molecule in the cell membrane. Retinal is shown in orange

Click here to read more about the vision mechanisms

Photomultipler tube Modern spectrometers make use of electronic devices (such as the photomultiplier tube) for detecting light. Electrons in the photocathode are so strongly "shaken" by light that some of them leave with a speed which depends of light frequency ("photoelectric effect").The electrical current crossing the tube (yellow) is further amplified by a sequence of secondary electron emission occurring at the impact of moving electrons with the dynode surfaces. In this process, additional energy is supplied to each electron by the voltage applied to each dinode (increasingly negative). Finally, current intensity allows one measuring the power of the incident light beam. Valence electrons in metals are not bound to energy quantization rules. More about the PMT.

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F.G. Banica, 2009-04-12