# Laser Physics: Comprehensive Study Notes 1. Coherence The concept of coherence is central to understanding laser physics. A wave is defined as being perfectly coherent if it appears as a pure sine wave for an infinitely long period of time or extends across an infinitely large space. In such an ideal wave, there is a definite, predictable relationship between the phase of the wave at a given time and a certain time later, or between a given point in space and a certain distance away. In reality, no actual light source emits a perfectly coherent wave. Real light waves are pure sine waves only for a limited period or in a limited space; thus, they are termed partially coherent waves . Coherence is broadly classified into two types based on the criteria of time and space: Temporal Coherence: This refers to the correlation of phase with time at a si...