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### What is diffraction

• It is a common observation with the waves of all kinds that they bend round the edge of an obstacle.

• Light like other waves also bends round the corner but in comparison to sound waves small bending of light is due to very short wavelength of light which is of the order of 10-5 cm.

• This effect of bending of light beams round the corner was first discovered by Grimaldi (1618-1663).

• We now define diffraction of light as phenomenon of bending of light waves around the corners and their spreading into geometrical shadow.

• Fresnel first explained that the diffraction phenomenon was the result of mutual interference between the secondary wavelets from the same wavefront.

• Thus we can explain diffraction phenomenon using Huygen's principle.

• The diffraction phenomenon is usually divided into two classes
1. Fresnel class of diffraction phenomenon where the source of light and scteen are in general at finite distance from the diffracting aperture
2. Fraunhofer class of diffraction phenomenon where source and the screen are at the infinite distance from the aperture , this is easily achieved by placing the source of light on the focal plane of a convex lens and placing screen on focal plane of another convex lens. This class of diffraction is simple to treat and easy to observe in practice.