Coherent Sources of Light

At any single point in space, at any instant, there are typically countless overlapping light waves arriving from many different sources all around -- by the superposition principle these must, technically, be interfering with one another at every instant. And yet, in ordinary everyday experience, no interference pattern is ever visible in ordinary room lighting. The reason is that different ordinary sources emit light of different FREQUENCIES, and even sources emitting the very same frequency are not, in general, in PHASE with one another -- so whatever interference pattern briefly forms at any instant keeps CHANGING, faster than any detector (including the human eye) can register, and no stable, visible pattern survives long enough to be seen.

To see a sustained, STEADY interference pattern, two light sources are needed that emit waves of the SAME frequency AND maintain a phase difference between them that stays CONSTANT in time (not necessarily zero -- just unchanging). Two such sources are called COHERENT sources. This condition cannot, in general, be satisfied by two entirely INDEPENDENT primary sources -- even if by chance they happen to emit the exact same frequency, their relative phase drifts randomly and uncontrollably over time, since each source's emission process is an independent, unrelated physical event. The only reliable practical solution is to derive TWO secondary sources from a SINGLE original primary source -- since both secondary sources are then, by construction, always exactly lo …