Why is light coherent?

[Lazarus]

The interference properties of light require the light to be coherent.

 

Since the origin of the light is state changes of individual atoms the

wave fronts (or individual photons) would seem to have random

spacing.

[mathematic]

Interference experiments involve light that is coherent, typically from one source. For example the double split experiment involves one source shining on both slits. The experiment wouldn't work if there were separate sources for each slit.

[Lazarus]

Interference experiments involve light that is coherent, typically from one source. For example the double split experiment involves one source shining on both slits. The experiment wouldn't work if there were separate sources for each slit.

 

But in the one source, indivdual attoms initiate the light, don't they?

[Strange]

Lasers generate coherent light (because the photon emission is stimulated by other photons).

[swansont]

Spatial coherence is achieved by having collimated light, so light from a single source at a reasonable distance will have enough spatial coherence for you to have interference. That ensures the phases of the photons have a fixed relation to each other and allow for the interference. Temporal coherence is achieved in lasers through stimulated emission, so the light is highly monochromatic and has the same starting phase.

 

We also know that photons interfere with themselves, so another way of seeing this is that incoherent light washes out the interference because minima and maxima of different photons will overlap without good collimation.

[Lazarus]

Spatial coherence is achieved by having collimated light, so light from a single source at a reasonable distance will have enough spatial coherence for you to have interference. That ensures the phases of the photons have a fixed relation to each other and allow for the interference. Temporal coherence is achieved in lasers through stimulated emission, so the light is highly monochromatic and has the same starting phase.

 

We also know that photons interfere with themselves, so another way of seeing this is that incoherent light washes out the interference because minima and maxima of different photons will overlap without good collimation.

 

Good explanation!