Question

Is a ray of light, produced by an ordinary light bulb, filtered through a perfect monochromatic filter and 2 (in-line) pinholes, coherent?

A physicist once told it is not, because even if it is perfectly monochromatic and perfectly collimated (by the two in-line pinholes), each photon has a completely random phase. For a while I thought this answered my question of what the difference between in phase and coherent really is.

But then I realized that the definition of coherence, which states that the phase-relation needs to be stable (over space and/or time), is fulfilled here: Any two photons have a stable phase relation at any point along the beam (and also any point in time). So what is then the difference between such a (hypothetical perfectly monochromatic and perfectly collimated) beam consisting exclusively of random phase-photons - and a laser beam, if a laser beam is coherent, but not consisting exclusively of in phase photons? The latter is what physicists have explicitely told me on several occasions: Lasers produce coherent light but the photons are not all in phase. (I really hope there was a misunderstanding because otherwise I see no possibility of grasping the issue.)

Answer 1

Even if light from an ordinary bulb is made monochromatic and collimated, it will not be coherent like laser light due to the absence of a stable phase relationship between photons. Laser coherence is achieved through the stimulated emission process, which ensures photons are emitted in phase and with the same frequency, a feature not provided by ordinary light sources.

Step-by-step explanation:

The question at heart is whether a ray of light, filtered through a perfect monochromatic filter and two pinholes, exhibits coherence similar to laser light, given that photons from an ordinary light bulb have random phases. Coherence in light waves denotes that they maintain a stable phase relationship with one another, which is true for laser light. However, an ordinary light source does not emit coherent light, even if it is monochromatic and collimated, because each photon maintains a random phase relation with the others, which prevents stable interference patterns and coherence.

Laser light achieves coherence through the process of stimulated emission, where photons are emitted in phase and with the same frequency due to population inversion in the lasing medium. This results in a cascade of photons that are phase-linked. Conversely, even perfectly filtered and collimated light from a conventional source lacks this mechanism to ensure that every photon is emitted with a stable phase relationship to the others, thus precluding true coherence.

Distinguishing between "in phase" and "coherence," laser light is coherent because the light waves in the laser beam not only share the same frequency but also maintain a stable phase relationship, which is not the case for light from ordinary sources through a filter and pinholes arrangement.