![]() After a complex biochemical process, a flow of nerve impulses is produced that is preprocessed by several layers of neurons in the retina that finally reach the visual cortex through the optic nerve, where the information is finally processed. The photoreceptor proteins are located in the cell membrane, so that when they absorb a photon they change shape, opening up channels in the cell membrane that generate a flow of ions. Response of photoreceptor cells of the human retina / Image: authorĮach of these types configures a type of photoreceptor cell in the retina, which due to its morphology are called cones. ![]() In fact, the human eye, for color vision, is endowed with three types of photoreceptor proteins, which produce a response as shown in the following figure. This requires receiving antennas of the order of hundreds of nanometres in size! But for nature this is not a big problem, as complex molecules can easily be this size. The human eye is sensitive to wavelengths ranging from deep red (700nm, nanometers=10 -9 meters) to violet (400nm). Structure of an electromagnetic wave / Image: authorīut before addressing this issue, it should be considered that to detect photons efficiently it is necessary to have a detector called an antenna, whose size must be similar to the wavelength of the photons. However, as we are going to see, this is not strictly physical but a consequence of the process of measuring and interpreting information, which makes color an emerging reality of another underlying reality, sustained by the physical reality of electromagnetic radiation. But it is common to use the term color as a synonym for frequency, since the color of light perceived by humans is a function of frequency. ![]() Since the photon can be modeled as a sinusoid traveling at the speed of light c, the length of a complete cycle is called the photon wavelength l, so the photon can be characterized either by its frequency or its wavelength, since l=c/n. ![]() Thus, we can find photons of very low frequencies located in the band of radio waves, to photons of very high energy called gamma rays, as shown in the following figure, forming a continuous frequency range that constitutes the electromagnetic spectrum. A rainbow is an optical and meteorological phenomenon that consists of the appearance in the sky of a multicolored arc of light, originated by the decomposition of sunlight in the visible spectrum / Image: pixabay The fundamental characteristic of a photon is its capacity to transfer energy in quantized form, which is determined by its frequency, according to the expression E=h∙n, where h is the Planck constant and n the frequency of the photon. The photon or “light quantum” was proposed by Einstein, for which he was awarded the Nobel Prize in 1921 and is one of the elementary particles of the standard model, belonging to the boson family. Visible light, heat, radio waves and other types of radiation all have the same physical nature and are constituted by a flow of particles called photons.
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