How does diffraction cause interference

[peter.ell]

Light normally doesn't interfere with itself because it is incoherent and unpolarized, right? So how is it that incoherent, unpolarized light passes through a circular aperture and interferes with itself?

How does the aperture suddenly cause the light to be able to interfere with itself? In essence, isn't light passing through a circular aperture the same as light being emitted from a circular source the same size? If so, then I would expect a small circular light source to show interference patterns, yet it doesn't, why not?

Thanks so much.

 

[Bill_K]

Light normally doesn't interfere with itself because it is incoherent and unpolarized, right? So how is it that incoherent, unpolarized light passes through a circular aperture and interferes with itself?

To get interference you need a very small aperture, the same order of magnitude as the wavelength of the light, so that each photon will be coherent across the aperture, even though the light beam as a whole is incoherent. Plus the beam needs to be monochromatic, so that each photon adds its contribution to the same interference pattern.

How does the aperture suddenly cause the light to be able to interfere with itself? In essence, isn't light passing through a circular aperture the same as light being emitted from a circular source the same size? If so, then I would expect a small circular light source to show interference patterns, yet it doesn't, why not?

Name an incoherent source as small as a wavelength of light? Better: consider a long wavelength example, like radio waves. Two antennas a wavelength or less apart, coherent with each other, will an exhibit interference pattern.