Stimulated emission is the key factor in Lasers and until 1917 this was not understood in science until Albert Einstein described how it would work, in theory. This was not proved until 1954 when American physicist Charles Townsend created an instrument that he called a population inversion device, this illustrated the fundamental outline of the modern laser.

American Physicist Theodore Mainman built the first true Laser in 1960. Scientists said it was more like " a solution looking for a question ".Lasers are the strongest source of light ever created by man. For example, a one milliwatt (1 mW) or one thousandth of a watt laser is about 1,000,000 times more brilliant than a 100-watt light bulb.  Lasers are grouped into Classes dependent on their output power.

The Class 1 laser is totally safe and has a very weak output. Class II is more powerful and can be found in Laser pointers. Class III are extremely powerful and can cut through materials. Class IV are the ultimate in laser power and can put a spot of light on the moon or weld metals.

Do not look into a Laser with your remaining eye. 

A laser is a device that produces a very narrow very powerful beam of light. Enough to drill 200 holes on a pin head or penetrate a diamond, the hardest material known to man. Laser beams can reach the moon with no loss of intensity. This of course depends on the size of the laser.

A Laser can be as long as a football pitch or as tiny as a grain of salt. they can be put to almost unlimited use including, playing music, reading barcodes, cutting and welding, guiding missiles, repairing damaged tissue, alignment of walls etc, printing and movement detection just to name a few.

A Laser consists of three basic elements. An energy source, an active medium and an optical cavity.

The energy source is any available device that provides energy to the active medium ( called " Pumping "). This can be electricity, another Laser or a flash lamp.

 The active medium can be for example a ruby or it can be a gas or a fluid that can store the energy and release it later as light. Mirrors at the ends of the medium bounce light back and forth to increase its magnitude. 

The optical cavity is where the light is focused and emitted. Also called a resonator it is the vessel that houses the medium.

There are four types of Laser. Solid-state, Semi-conductor, gas and dye.

Laser light emits from excited atoms in the given active medium, for example a crystal. Normally atoms are at a low level of excitement called " ground state". Atoms change from this dormant state to an excited state when they are stimulated by absorbing various types of energy called "absorption". This can be produced by pulsing ultra bright light through the active medium for example to get the desired result of light.

Atoms in lasers generally absorb light energy  known as photons. The excited atom holds extra energy milliseconds before releasing it as another photon, it falls back into its ground state and repeats the process over and over again, millions of times a second. Called " spontaneous emission"

The excited state of some atoms can store energy for an atomically long time, 1/1,000 of a second being the norm. When the photon hits the atom for this period of time it will give off ( emit) an identical atom. Called " Stimulated emission" hence light will emit from the laser in a single narrow beam.

Stimulated emission is the process that a laser uses. One stimulated photon produces another photon. This of course is doubling the quantity, called " amplification". These atoms keep on doubling and rebounding off  mirrors at the ends of the active medium. At one end of the medium the mirror is only partially coated and this is the end that the light eventually (in millionths of a second) force their way out. This happens because each photon is producing a chain reaction of stimulated emissions within the medium.

Laser light is different from say the light of a torch in that it has low divergence. (It doesn't spread out.) And the other property is that it is monochromatic ( single coloured) Therefore the light is coherent.

Light is basically a collection of electromagnetic wave forms and its colour is determined by its wavelength, this is usually measured in angstroms. Ordinary light has many wavelengths and because they are all jumbled up together incoherently, we see it as white light. To filter out all of the present colour's so that we get a single red light for example we can put a red transparent glass in front of the light, but even though we can do this the light waves are still NOT coherent.

Laser light only has a couple of wavelengths, and they are all narrow  with no exceptions. Hence Laser light is only ever seen in one colour at a time. A red laser light or a blue laser light for example, will only produce red or blue respectively.

The Light from a Laser is coherent because it is  100% in phase with itself and the light waves are coordinated so that they will only travel in a straight line. They do not differ at all, unlike normal light that moves in irregular patterns with all sorts of wavelengths in many different directions.