Light is a very unique phenomenon that is spread all over our universe. Its source in the universe is the electromagnetic radiation that is emitted by the gaseous stars known to us as black body radiation. Light speed is a reference value for our life on this planet since it is the limiting speed that our capabilities cannot reach except for light itself. Light velocity is constant in all inertial reference systems. This is one of the postulates of the theory of special relativity.
Light can be affected by gravitational forces as is predicted by the general theory of relativity. Usually light travels in the universe in a straight line trajectory. The theory of general relativity predicts correctly that when light passes near a star with large gravitational field, this light will be deflected by the gravitational force of that star and will be deflected from its trajectory in a bent manner.
Light is unique because it does not have a mass. Even so it is affected by the gravity that can affect its photons frequency or its wavelength. Light was thought, in early times, to be an electromagnetic phenomenon with purely wave characters. Experiments that were conducted in the early 19th century on the properties of light showed that light can behave like waves, as expected, but also other experiments showed that light behaves like particles.
Experiments that were conducted on light diffraction and light interference showed that light has wave properties. On the other hand, experiments such as the photoelectric effect and the Compton effect showed that light behaves like a particle. Einstein called this particle of light a photon. This photon associates with it an energy which satisfies the following equation:
E=h*(ni) where E is the energy of the photon and h is planck constant and ni is the frequency of the light.
Electromagnetic light has a periodic form that is manifested as sinus and cosinus periodic functions. Electromagnetic wave has associated with it an electric as well as a magnetic field. In a polarized light the electric and the magnetic fields are perpendicular to each other. The direction in which the wave propagates is perpendicular to both the electric and the magnetic fields.
As was mentioned previously a light particle or a photon does not have a mass associated with it. It does not have a rest energy according to the theory of special relativity. Its energy is related to its momentum by the following equation:
E=pc where E is the energy of the photon and p is the momentum that is associated with it. C stands for the speed of light.
Light or electromagnetic radiation can be generated in the universe from the electromagnetic radiation of very hot stars. This electromagnetic radiation can give us information about the structure of that star such as its atomic composition. In the laboratory light can be generated from electronic transitions in atoms and molecules. Each atom and molecule has distinct light emission frequencies that are characteristic of that atom or molecule.
Very energetic electromagnetic radiation can be obtained in nuclear reactions in which case it is called gamma radiation. This radiation is very harmful to humans due to its ability to break chemical bonds in DNA molecules. This is so due to the highly energetic photons that this radiation carries. Also X-ray radiation can be obtained from the expulsion of an electron in an atom that is located near the nucleus in the most inner orbital of the atom.
X-ray is also harmful to humans that are exposed to it due to its ability to break chemical bonds in DNA molecules in the human body. X-ray is used often in cancer therapy due to its ability to kill cancer cells. However a side effect of this treatment is the destruction of the good cells also.
UV radiation is another type of light that is also harmful to humans due to its mutagenic potential, namely, its ability to trigger cancer. Fortunately, the ozone layer in the upper atmosphere can react with the UV light and prevent it from reaching the earth surface. An application of light in all its available wavelengths is the LASER and the MASER. LASER is a monochromatic light. Namely, it is of only one wavelength. In addition, it has only one phase. LASER is short of light amplification by stimulated emission radiation.
The principle of LASER action is as follows:
An excitation of an electron from a singlet state of the ground state of the atom to an excited singlet state that is designated as singlet 2 will be followed by an intersystem crossing from the singlet state 2 to a triplet state that is energetically very similar to that of the singlet 2 state. The transition from the triplet state 2 to the ground state is forbidden. Therefore, stimulating this process by shining a light photon on the electron in the triplet state will lead to its stimulated going to the singlet state 1 with the concomitant release of 2 photons. This is shortly the process of light amplification in LASERS.