It is a matter of common experience that the sound waves or water escaping through a small hole spread out in all directions as if they originated at the hole. After passing through the hole all waves do not propagate in its original direction but a part of it is bent, the wave fronts for the ripples being semicircles with their centres situated at the centre of the hole. This phenomenon is called "diffraction" and is an essential characteristic of all wave motions.
In a similar way, when a beam of light passes through a small opening, it also deviated from its rectilinear path and does bend round the corner as other types of waves do but the amount of this bending is extremely small. The small bending in case of light is due to its very small wavelength of the order 6000A. The wavelength of ordinary sound waves is nearly 60cm. Consequently the diffraction in case of light is .000001 fold less, a small obstacle will obstruct the path of light whereas a much lager obstacle will be required to achieve the same effect in sound.
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