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laser, a device that stimulates atoms or molecules to emit light at particular wavelengths and amplifies that light, typically producing a very narrow beam of radiation. The emission generally covers an extremely limited range of visible, infrared, or ultraviolet wavelengths. Many different types of lasers have been developed, with highly varied characteristics. Laser is an acronym for “light amplification by the stimulated emission of radiation.”

History
The laser is an outgrowth of a suggestion made by Albert Einstein in 1916 that under the proper circumstances atoms could release excess energy as light—either spontaneously or when stimulated by light. German physicist Rudolf Walther Ladenburg first observed stimulated emission in 1928, although at the time it seemed to have no practical use.

In 1951 Charles H. Townes, then at Columbia University in New York City, thought of a way to generate stimulated emission at microwave frequencies. At the end of 1953, he demonstrated a working device that focused “excited” (see below Energy levels and stimulated emissions) ammonia molecules in a resonant microwave cavity, where they emitted a pure microwave frequency. Townes named the device a maser, for “microwave amplification by the stimulated emission of radiation.” Aleksandr Mikhaylovich Prokhorov and Nikolay Gennadiyevich Basov of the P.N. Lebedev Physical Institute in Moscow independently described the theory of maser operation. For their work all three shared the 1964 Nobel Prize for Physics.

An intense burst of maser research followed in the mid-1950s, but masers found only a limited range of applications as low-noise microwave amplifiers and atomic clocks. In 1957 Townes proposed to his brother-in-law and former postdoctoral student at Columbia University, Arthur L. Schawlow (then at Bell Laboratories), that they try to extend maser action to the much shorter wavelengths of infrared or visible light. Townes also had discussions with a graduate student at Columbia University, Gordon Gould, who quickly developed his own laser ideas. Townes and Schawlow published their ideas for an “optical maser” in a seminal paper in the December 15, 1958, issue of Physical Review. Meanwhile, Gould coined the word laser and wrote a patent application. Whether Townes or Gould should be credited as the “inventor” of the laser thus became a matter of intense debate and led to years of litigation. Eventually, Gould received a series of four patents starting in 1977 that earned him millions of dollars in royalties.

first laser
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first laserTheodore H. Maiman of Hughes Aircraft Company showing a cube of synthetic ruby crystal, the material at the heart of the first laser.
first gas laser
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first gas laserAli Javan, a researcher at Bell Telephone Laboratories, displaying the first gas laser, which used a mixture of helium and neon.
The Townes-Schawlow proposal led several groups to try building a laser. The Gould proposal became the basis of a classified m

1. Magnetism in matter Magnetization Vector, M and Magnetic Field Strength H   The magnetic state of a substance is described by a quantity called the magnetization vector M . The magnitude of this vector is defined as the magnetic moment per unit volume of the substance (magnetic moment /cm 3 ).   Magnetic moment , is also called a  magnetic dipole moment (Ampere)(meter) 2 , is a measure of the object's tendency to align with a magnetic field. Its positive direction depends on the way the object responds to the magnetic field.  

2. The total magnetic field Induction at a point within a substance depends on both the applied (external) field and the magnetization M of the substance. Recognizing the similarity between M and H, we can write: B = µ o ( H+ M ) µ o is a constant called the permeability of free space = 4πx10 -7 (T. m/A)  H magnetic field strength within the substance (A/m).   The quantities H and M have the same units. Because M is magnetic moment per unit volume, its SI units are: (Ampere)(meter) 2 /(meter) 3 = ( A/m ).   So, for these substances placed in an external magnetic field, we can write: M = χ H   where χ (Greek letter Chi) is a dimensionless factor called the magnetic susceptibility.

3. Classification of Magnetic Substances   Substances can be classified as belonging to one of 3 categories, depending on their magnetic properties: Paramagnetic , Ferromagnetic and Diamagnetic   The two most common types of magnetism are paramagnetic and Diamagnetic, which account for the magnetic properties of most of the periodic table of elements at room temperature. Paramagnetic and ferromagnetic materials are those made of atoms that have permanent magnetic moments. Diamagnetic materials are those made of atoms that do not have permanent magnetic moments . The magnetic properties of a substance can be  determined  by examining  its electron configuration:  If it has  unpaired electrons, then the substance is paramagnetic  and  if  all electrons are paired, the substance is then  diamagnetic . 

4. Ferromagnetism   A small number of crystalline substances exhibit strong magnetic effects called ferromagnetism.   Some examples of ferromagnetic substances are iron, cobalt, nickel, gadolinium, and dysprosium. These substances contain permanent atomic magnetic moments that tend to align parallel to each other even in a weak external magnetic field. Once the moments are aligned, the substance remains magnetized after the external field is removed. This permanent alignment is due to a strong coupling between neighboring moments  

5. Paramagnetism   Paramagnetic substances have a small but positive magnetic susceptibility resulting from the presence of atoms (or ions) that have permanent magnetic moments. These moments interact only weakly with each other and are randomly oriented in the absence of an external magnetic field. When a paramagnetic substance is placed in an external magnetic field, its atomic moments tend to line up with the field. However, this alignment process must compete with thermal motion, which tends to randomize the magnetic moment orientations.   Diamagnetism   When an external magnetic field is applied to a diamagnetic substance, a weak magnetic moment is induced in the direction opposite the applied field. This causes diamagnetic substances to be weakly repelled by a magnet. Although diamagnetism is present in all matter, its effects are much smaller than those of paramagnetic or ferromagnetic.