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Physically, light (including X-rays, ultraviolet, visible light, infrared, radio) is an electromagnetic wave and always appears in the form of finite quanta, photons. But what do these terms mean?
Cosmology Series
  1. What is Cosmology?
  2. The Speed of Light
  3. Redshift and Blueshift
  4. What is the Cosmic Microwave Background Radiation?
  5. Cosmic Scale Factor and Expansion of the Universe
  6. The Physics Governing the Universe
  7. The Nature of Light
  8. The Four Forces in the Universe
  9. The Expansion of the Universe

In an atom, there are a certain number of electrons, protons and neutrons. The protons and neutrons exist together in the nucleus, while the electrons "orbit" the nucleus at various distances. These distances are called electron levels or quantum states and are typically referenced as n. This is a simplified explanation of the Bohr model.

electron-levels

electron-levels

When an atom is excited, electrons jump from one level to a higher level, and each transition of an electron corresponds to the emission of one photon from the atom. This photon forms an electromagnetic wave.

The energy, E, of a photon is given by the following formula.

The energy of a photon
Equation 46 - The energy of a photon

Where h is Planck's constant: 6.63x10-34 J s, v is the frequency of the wave, c is the speed of light and λ is the wavelength of the electromagnetic wave.

The emitted photon has the same energy as the energy difference between the two quantum states in the atom.

When atoms are excited, electrons move from a lower to higher quantum state releasing a photon as they do. For every electron in an atom, there is a certain threshold energy, such that if this energy is transferred to the electron, it can leave the atom. The remaining atom is positively charged and called a positive ion. This process is known as ionisation. Every further electron removed from the atom increases the level of ionisation of this atom. If a neutral atom gains an additional electron it becomes a negative ion and if it loses an electrum it becomes a positive ion.

If the electromagnetic wave loses energy, its wavelength becomes longer and the frequency lower. The wavelength may be altered in many circumstances. If the wavelength increases, we say it is redshifted; if it decreases, we say it is blue-shifted. Redshift and Blueshift are particularly useful for determining the speed and acceleration of distant objects.

Wave-particle duality is the concept that the electrons in the electromagnetic wave are both particles and waves at the same time, although paradoxically when measured it is only one of the two.

The phenomenon of diffraction is a well-known property of light waves. At the beginning of the 20th century, a problem was found with the theories of light waves emitted from hot objects, such as light from the sun. This light is called black-body radiation. These theories would always predict infinite energy for the light emitted beyond the blue end of the spectrum, and this is a contradiction of the principles of conservation of energy. Clearly, a new model for the behaviour of black bodies was needed.

The answer was to assume the energy of light waves was not continuous but came in fixed amounts as if it was composed of a large number of particles, or photons.

The strange thing about the diffraction experiment is that the electron wave doesn't deposit energy evenly over the entire surface of the detector, as you might expect with a sea wave crashing on the shore. Instead, the energy of the electron is deposited in points, just as if it was a particle. So while the electron propagates through space like a wave, it interacts at a point like a particle. This is known as wave-particle duality.

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