Diffraction of Light

The property of bending of light around obstacles and spreading of light waves into the geometrical shadow of an obstacle is called diffraction. 
If the shadow of an object cast on a screen by a small source of light is examined, it is found that the boundary of the shadow is not sharp. The light is not propagated strictly in straight lines. This phenomena of diffraction, which occurs as the light passes the object, results from the wave nature of light. Banded or annular patterns, diffraction patterns, are produced near the edges of the shadow. Small apertures in objects produce a similar effect.  

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      In Young's double slit experiment for the interference of light, the central region of the fringe system is light. If light travels in a straight line, the central region should appear dark i.e. the shadow of the screen between the two slits. Another simple experiment can be performed for exhibiting the same effect.
       Consider that a small and smooth steel ball of about 3mm in diameter is illuminated by a point source of light. The shadow of the object is received on a screen as shown in the figure given below. The figure shows that the shadow of the spherical object is not completely dark but has a bright spot at its center. According to Huygen's principle, each point on the rim of the sphere behaves as a source of secondary wavelets which illuminate the central region of the shadow.

Bending of light caused by its passage past a spherical object

    These two experiments clearly show that when light travels past an obstacle, it does not proceed exactly along straight path, but bends around the obstacle.
       The phenomena is found to be prominent when the wavelength of light is large as compared with the size of the obstacle or aperture of the slit. The diffraction of light occurs, in effect, due to the interference between rays coming from different parts of the same wavefront.

                    Their colors are due to diffraction                   .