Light, the portion of electromagnetic radiation that the human eye can see. To be seen, light must have a wavelength between 400 and 750 nanometers, a range known as the visible spectrum. The eye recognizes light of different wavelengths as being of different colors, the shorter wavelengths forming the blue end of the visible spectrum, the longer the red. The term light is also applied to radiations of wavelengths just outside the visible spectrum: those of energies greater than that of visible light are called ultraviolet light, and those of lower energies are called infrared. For many years the nature of light aroused controversy among physicists. Although Christiaan Huygens had demonstrated that reflection and refraction could be explained in terms of waves (1690), Isaac Newton preferred to think of light as composed of material corpuscles, or particles (1704). Thomas Young's interference experiments reestablished the wave hypothesis (1801) and A. J. Fresnel gave it a rigorous mathematical basis (1814–15). At the beginning of the 20th century, the nature of light was again debated as Max Planck and Albert Einstein proposed explanations of blackbody radiation (1900) and the photoelectric effect (1905) respectively, both of which assumed that light comes in discrete quanta (bundles) of energy.