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Coherent light means monochromatic light and alle waves have the same phase difference. This is given for laser, where the resonator is a potential box and the outgoing waves have the same phase difference. What is about a thermal source like a bulb? The emitting surface is not smooth, the involved electrons are not swinging in phase. So, how it is possible to produce coherent light with a thermal source?

The background for this question: https://physics.stackexchange.com/questions/183935/why-the-distance-between-the-light-source-and-the-slits-screen-seems-to-be-a-po So the question is about coherence in front of any slit or edge.

Community
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HolgerFiedler
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Coherency of light in practice is not an either/or issue. Any light due to any source has some degree of coherence. Laser light has usually much higher coherence than light of a hot metal filament.

Some degree of coherence means, in simple wording, that light waves at one point of space due to different parts of the source behave similarly (they have non-zero covariance). For thermal source, the lower the distance between two of its parts, the greater the covariance of their retarded fields at the observation point.

In usual circumstances the coherence of light due to thermal sources is very low and interference effects are hard to observe. It can be increased, however, with color filter and a diaphragm with a narrow slit/hole placed in the path of the light before it hits the measuring instrument.

Ján Lalinský
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