Why does light diffract only through slits?

We can see diffraction of light if we allow light to pass through a slit, but why doesn't diffraction occur if we obstruct light using some other object, say a block? Why are shadows formed? Why doesn't light diffract around the obstruction as it does around the slit?

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"Why doesn't light diffract around the obstruction as it does around the slit?". But it does. See for instance Babinet's principle – Trimok Aug 13 '13 at 11:07
Light diffract around obstructions. If someone explains mechanism of diffraction, that will also explain why it is not supposed to bend ONLY around slits, but rather, any type of obstruction. – kpv Apr 5 at 21:20

Light will diffract around anything, including a block. We simply don't observe this diffraction because most blocks are very large compared to the wavelength of light. One historically-famous example of diffraction around an object is Poisson's spot, in which diffraction around a circular disk is easily observed.

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Good question. Actually, early 19th century this question was put forward as an argument against diffraction theory.

The below is from http://minerva.union.edu/jonesc/scientific_photos%202010.htm :

"In 1818, Fresnel presented his theory explaining diffraction of light. Poisson denied the truth of the theory, and argued that a consequence of the theory would be that there would be a bright spot right in the middle of the shadow of a circular object. The spot, shown above of the shadow of a ball bearing suspended on a needle, was promptly discovered by Arago. This was a lot harder to do before lasers were invented!"

Note: observation of such diffraction patterns are best done in (nearly) monochromatic light. Hence the remark about using lasers. The bright spot at the center of a circular shadow is known as an Arago spot.

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also often called Poisson's spot, in honor of his mathematical reasoning! – Peter Diehr Apr 5 at 21:06

Light diffracts through any object but the wavelength must be greater when compared to the size of the object. In this case it is slits so.

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Hi Nike, you have the start of a good answer here. You should expand it a bit to provide more details. – Brandon Enright Dec 2 '14 at 5:45

Diffraction can also occurr around abstruction . Suppose u put ur finger infront of light w.front such that ur fingers shadow will appear on the screen placed behind ur finger . When light w.front strikes ur finger then the light ray at the upper and lower extremes of obstruction will bend and enter in to shadow region generated by the obstruction and in this case light will not spread rather it will converg into shadow region and form argo spot

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