We live immersed in electromagnetic fields, surrounded by radiation of natural origin or produced by artifacts made by humans. This radiation has a dual behavior of wave and particle, where the particles can be considered as packets of electromagnetic waves. Waves are characterized by their wavelength, the distance between two consecutive peaks. Of all the radiation that continuously reaches our eyes, we are only able to see (i.e. our retina photoreceptors are only sensitive to) electromagnetic radiation with wavelengths within the range of 380nm to 740nm (a nm or nanometer is one-billionth of a meter). We are not able to see radiation outside this band, such us ultraviolet radiation (wavelength of 10nm to 400nm) or FM radio (wavelengths near 1m). Therefore, light is defined as radiation with wavelengths within the visible spectrum of 380nm to 740nm. Figure 1.1 shows the full spectrum of radiation with a detail of the visible light spectrum. The sun emits full-spectrum radiation, including gamma rays and ultraviolet and infrared “light,” which of course have an effect on our bodies even if we are not able to see them.