The term laser is an acronym for Light Amplification by Stimulated
Emission of Radiation. It refers to the production of a beam of radiation, which differs
from ordinary light in the following ways:
Lasers are of a single specific wavelength and hence of a defined
frequency. In the case of visible lasers a single pure colour is produced, e.g. ruby
lasers give a red light at 694.3 nm. Single – wavelength laser radiation is also referred
to as monochromatic in the infrared and ultraviolet regions, despite being invisible.
Laser radiation is not only of the same wavelength but also in phase, i.e.,
the peaks and troughs of the electric and magnetic fields all occur at the same time.
This is called temporal coherence. The laser radiation travel in the same direction;
this is called spatial coherence.
The distance over which the wavelengths stay in phase is called the
coherence length. It varies from less than a millimeter to hundreds of metres.