harve

Provided that every light source is multiple consists of milions atoms emiting photons,how the unique light source used is considered to be coherent?

Jerromyjon

The only thing that needs to be coherent is the combination of the colliding waves of photons moving through both slits.
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harve

Yes.Since the source is multiple (composite) the two separated photonbeams cannot be coherent.

topsquark

Two beams can't, but you don't have two beams. There will be a huge number of photons being emitted through what is essentially a diffraction grating with the spacing of an atomic separation. What comes out is a very large (compared to the scale of the atom) central "beam." Coherency is a bit more difficult and depends on how the light is generated. Are you asking about a coherent beam? Or a monochromatic beam with no diffraction pattern?

-Dan

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harve

Yes,I'm asking about a coherent beam.Is this a laser beam?But this is monochromatic.Are there such beams without patterns?

MBW

Going back to the walking people analogy;
"normal" light could be likened to a crowd of people leaving a stadium,
there is a lot of randomness in the individual photon energies, etc...
Coherent laser light would then be like an army marching out of a barracks,
The individual photons are almost identical, and "marching" in phase.

As you indicate, laser light is monochromatic light,
thus a diffaction grating will not produce a spectrum.
If you were to combine the output of several lasers of different colours into a single beam,
then the difracfraction grating would re-separate the different coloured beams.

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harve

Has a monochromatic beam no diffraction pattern? Why?

MBW

Sorry I think I confused the issue unnecessarily,
Monochromatic light will produce a diffration pattern.

Different wavelengths (colours) of light will be diffracted by different amounts by the same diffraction grating.
Monochromatic light will therefore actually give a clearer pattern than multi-chromatic white light.

Coherence is not necessary for diffraction.

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