Diffraction: The bending of light round corners and spreading it into the regions of geometrical shadows as it passes through an aperture or across an obstacle in their path is called diffraction of light.
When a narrow slit AB is placed in path of the light, then only part A'B' of the screen should be illuminated and no light should enter the regions A'X and BY of the screen Fig. (a). On the other hand, when obstacle AB is placed, then its distinct geometrical shadow A'B' should be obtained Fig. (b)
It happens exactly so only when size of slit or obstacle is large. However, if the size of the slit or obstacle is made small, then light enters in the geometrically prohibited regions showing thereby that the light bends round the corners of the slit or obstacle.
This phenomenon of bending of light round the corners and spreading into the regions of the geometrical shadow is called diffraction.
In principle, all the types of wave motion exhibit diffraction effect. The diffraction of radio waves and sound waves is readity observed in practice.
