Diffraction:

You might have experienced the formation of waves in a pond of still water while disturbed by throwing a stone. Have you ever seen the modification in such a wave while it encounters a small barrier or a narrow opening? If you have observed, you would recall that these waves bend or change their direction of propagation on meeting a barrier or an opening. That phenomenon of bending of a wave round the corners of sharp barriers or by narrow openings is known as diffraction. Electromagnetic radiations also show this phenomenon. The consequence of diffraction could be observed through placing a photographic plate within the path of diffracted radiation. The occurrence of a pattern of alternate bright and dark bands is characteristic of the diffraction. The intensity is maximum at the central band and reduces gradually as the distance rise and not visible beyond certain distance. The presence of light in the areas that are otherwise expected to be dark accounts for the bending of light.

Figure: Diffraction of monochromatic radiation through a narrow slit and the resulting intensity pattern

The diffraction phenomenon is a consequence of interference of wavelets originating from different parts of the slit. The inner corners as well as the central elements of the slit act as the sources of new wavelets that interfere to provide the observed intensity patterns.

The extent of diffraction through narrow openings (slits) depends inversely on the width of the slit i.e., narrower a slit greater the diffraction. The effect becomes observable when the size of the slit is comparable to the wavelength of the light.