There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. At the lower end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule retinal in the human retina, which change triggers the sensation of vision. When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.ĮMR in the visible light region consists of quanta (called photons) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths. The behavior of EMR depends on its wavelength. The designation " radiation" excludes static electric, magnetic and near fields. Generally, electromagnetic radiation (EMR) is classified by wavelength into radio waves, microwaves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays and gamma rays. With the development of electric lights and power systems, electric lighting has effectively replaced firelight.Įlectromagnetic spectrum and visible light The electromagnetic spectrum, with the visible portion highlighted Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. The main source of natural light on Earth is the Sun. The study of light, known as optics, is an important research area in modern physics. Like all types of electromagnetic radiation, visible light propagates by massless elementary particles called photons that represents the quanta of electromagnetic field, and can be analyzed as both waves and particles. Its speed in vacuum, 299 792 458 m/s, is one of the fundamental constants of nature. The primary properties of light are intensity, propagation direction, frequency or wavelength spectrum and polarization. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. In physics, the term "light" may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahertz, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). Light or visible light is electromagnetic radiation that can be perceived by the human eye.
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