question: how can light have energy?
comment: e = mc2 is not the full equation E²=(mc²)²+(pc)² Is the full one. Essentially, with m=0, you can remove that part of the equation, and end up with E= pc (with p being momentum, and light, since it cannot be at rest, has momentum) This would apply to anything "mass less" such as a photon, as the mc2 part is zero, so you're left with the pc part only.
passages: Photon energy is transferred to matter in a two-step process. First, energy is transferred to charged particles in the medium through various photon interactions (e.g. photoelectric effect, Compton scattering, pair production, and photodisintegration). Next, these secondary charged particles transfer their energy to the medium through atomic excitation and ionizations.
   It is well known that light, or more generally an electromagnetic wave, carries not only energy but also momentum, which is a characteristic property of all objects in translational motion. The existence of this momentum becomes apparent in the “"radiation pressure"” phenomenon, in which a light beam transfers its momentum to an absorbing or scattering object, generating a mechanical pressure on it in the process.
   Section::::Light.
   Photon energy
  
  Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy.
   As a wave, light is characterized by a velocity (the speed of light), wavelength, and frequency. As particles, light is a stream of photons. Each has an energy related to the frequency of the wave given by Planck's relation "E = hf", where "E" is the energy of the photon, "h" = 6.626 × 10 J·s is Planck's constant, and "f" is the frequency of the wave.
   There are many sources of light. A body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is sunlight, the radiation emitted by the chromosphere of the Sun at around peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units and roughly 44% of sunlight energy that reaches the ground is visible. Another example is incandescent light bulbs, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames, but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum.
   Section::::Physics.:Wave–particle duality.
  
  The modern theory that explains the nature of light includes the notion of wave–particle duality. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned using an object with a large mass. A bold proposition by Louis de Broglie in 1924 led the scientific community to realize that electrons also exhibit wave–particle duality.
  
  Section::::Physics.:Wave and particle effects of electromagnetic radiation.
   Section::::Scientific use.:Biology.
   Nri people believed that the sun was the dwelling place of Anyanwu (Light) and Agbala (Fertility). Agbala was the collective spirit of all holy beings (human and nonhuman). Agbala was the perfect agent of Chukwu or Chineke (the Creator God) and chose its human and nonhuman agents only by their merit; it knew no politics. It transcended religion, culture and gender, and worked with the humble and the truthful. They believed Anyanwu, The Light, to be the symbol of human perfection that all must seek and Agbala was entrusted to lead man there.
  
  Section::::Tradition.
   BULLET::::- Energy also has mass according to the principle of mass–energy equivalence. This equivalence is exemplified in a large number of physical processes including pair production, nuclear fusion, and the gravitational bending of light. Pair production and nuclear fusion are processes in which measurable amounts of mass are converted to energy, or vice versa. In the gravitational bending of light, photons of pure energy are shown to exhibit a behavior similar to passive gravitational mass.
   Sunlight also drives many weather phenomena on Earth. One example is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, give up some of their thermal energy suddenly to power a few days of violent air movement. Sunlight is also captured by plants as chemical potential energy via photosynthesis, when carbon dioxide and water are converted into a combustible combination of carbohydrates, lipids, and oxygen. The release of this energy as heat and light may be triggered suddenly by a spark, in a forest fire; or it may be available more slowly for animal or human metabolism when these molecules are ingested, and catabolism is triggered by enzyme action.
   O*is excited oxygen – meaning, one or more electrons in the O molecule have been promoted to higher-energy molecular orbitals. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the ground state by lowering its energy. It can do that in more than one way:
  
  BULLET::::- it can react further, without any light emission
  
  BULLET::::- it can lose energy without emission, for example, giving off heat to the surroundings or transferring energy to another molecule
  
  BULLET::::- it can emit light
   Section::::Background in economics.:Original model.
   The Maxwell wave theory, however, does not account for "all" properties of light. The Maxwell theory predicts that the energy of a light wave depends only on its intensity, not on its frequency; nevertheless, several independent types of experiments show that the energy imparted by light to atoms depends only on the light's frequency, not on its intensity. For example, some chemical reactions are provoked only by light of frequency higher than a certain threshold; light of frequency lower than the threshold, no matter how intense, does not initiate the reaction. Similarly, electrons can be ejected from a metal plate by shining light of sufficiently high frequency on it (the photoelectric effect); the energy of the ejected electron is related only to the light's frequency, not to its intensity.
   In a similar chain of transformations beginning at the dawn of the universe, nuclear fusion of hydrogen in the Sun releases another store of potential energy which was created at the time of the Big Bang. At that time, according to one theory, space expanded and the universe cooled too rapidly for hydrogen to completely fuse into heavier elements. This resulted in hydrogen representing a store of potential energy which can be released by nuclear fusion. Such a fusion process is triggered by heat and pressure generated from the gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into starlight. Considering the solar system, starlight, overwhelmingly from the Sun, may again be stored as gravitational potential energy after it strikes the Earth. This occurs in the case of avalanches, or when water evaporates from oceans and is deposited as precipitation high above sea level (where, after being released at a hydroelectric dam, it can be used to drive turbine/generators to produce electricity).
   Section::::Scientific use.:Cosmology.
   Tetsuro Araki, the director of the anime, said that he felt an urge to support and cheer for Light. Araki added that Light would have used and killed him if he was one of Light's friends, but the director still believed that Light is ""that" interesting" and therefore he would have felt an attraction towards Light.
   where "h" is a numerical value called Planck's constant.
  
  Then, Albert Einstein in 1905, in order to explain the photoelectric effect previously reported by Heinrich Hertz in 1887, postulated consistently with Max Planck's quantum hypothesis that light itself is made of individual quantum particles, which in 1926 came to be called photons by Gilbert N. Lewis. The photoelectric effect was observed upon shining light of particular wavelengths on certain materials, such as metals, which caused electrons to be ejected from those materials only if the light quantum energy was greater than the work function of the metal's surface.
   Section::::History.
   Section::::Physical properties.
   Anything that can transmit energy can have an intensity associated with it. For a monochromatic propagating electromagnetic wave, such as a plane wave or a Gaussian beam, if "E" is the complex amplitude of the electric field, then the time-averaged energy density of the wave, travelling in a non-magnetic material, is given by:
  
  and the local intensity is obtained by multiplying this expression by the wave velocity, c/"n":
  
  where "n" is the refractive index, c is the speed of light in vacuum and formula_7 is the vacuum permittivity.
   While Einstein was the first to have correctly deduced the mass–energy equivalence formula, he was not the first to have related energy with mass. But nearly all previous authors thought that the energy that contributes to mass comes only from electromagnetic fields.
  
  Section::::History.:Newton: matter and light.
  
  In 1717 Isaac Newton speculated that light particles and matter particles were interconvertible in "Query 30" of the "Opticks", where he asks:
  
  Section::::History.:Swedenborg: matter composed of "pure and total motion".
   Photon energy can be expressed using any unit of energy. Among the units commonly used to denote photon energy are the electronvolt (eV) and the joule (as well as its multiples, such as the microjoule). As one joule equals 6.24 × 10 eV, the larger units may be more useful in denoting the energy of photons with higher frequency and higher energy, such as gamma rays, as opposed to lower energy photons, such as those in the radio frequency region of the electromagnetic spectrum.
  
  Section::::Formula.
  
  The equation for photon energy is
   Section::::Scientific use.:Quantum mechanics.
  
  In quantum mechanics, energy is defined in terms of the energy operator
   Section::::Characteristics.
  