Why are there interference patterns from black body radiation?

Question

As it turns out, descriptions of this are already all over the internet:

• http://physicslens.com/single-slit-diffraction-using-fingers/
• What are the lines visible between two cards held edge-to-edge?
• https://www.reddit.com/r/askscience/comments/4yolbi/when_i_hold_two_fingers_together_and_look_through/d6pp2ft/ (direct link to a very nice answer)

I don't think describing the procedure is necessary, just click on any of the links.

I tried it both with my fingers and with cards a lot and it seems to always work. I only tried artificial light sources, not sun light, for safety reasons.

However, isn't the light of incandescent light bulbs (I – among other kinds of light sources – tried it with several different ones of those) black body radiation which isn't coherent whatsoever? Why does it still work?

I'm not using any optical instruments (glasses, contact lenses, prisms, etc.), btw. Just a light source, my eyes, my fingers, and cards.

Answer 1

Black body radiation and any large in spatial extent source radiation is incoherent , true.

Point sources by their construction create coherence.

This illustrates space coherence

Incoherent light going through a small hole creates coherence, and that is how Young got coherence for the double slit experiment.

When light is scattered off an obstacle, as the edge of a paper, or the slit between fingers, the reflecting points on edge act like a point source creating spatial coherence in the plane perpendicular to the edge.

So coherence does not come from the black body radiation of the source, but from the geometry at the level of scattered light and the dimensions involved

Answer 2

Visible light is only a portion of the black body radiation spectrum and these frequencies are a small enough subset that the modes of wave propagation are visible through the slit. (I am avoiding the word interference as the phenomenon is really a result of the photon as a wave function.)