Single Slit Diffraction

Huygens[1] proposed that every point to which a luminous disturbance reaches becomes a source of a spherical wave, and the sum of these secondary waves determines the form of the wave at any subsequent time.

Huygens assumed that the secondary waves traveled only in the "forward" direction and it is not explained in the theory why this is the case. He was able to provide a qualitative explanation of linear and spherical wave propagation, and to derive the laws of reflection and refraction using this principle.

Diffraction is a phenomenon whereby waves appear to bend around obstacles, or appear to spread out after passing through a small orifice. The occurrence of diffraction allows us, for example, to hear sounds from sources that are hidden from us by some obstacle or other.

Diffraction is a wave phenomenon and is also observed with water waves in a ripple tank.

A wave spreads out (noticeable diffraction) when the size of the slit is comparable to or smaller than the wavelength

When wave passes through a small opening, comparable in size to the wavelength, in an obstacle (in this case yellow rectangles) , the wavefront on the other side of the opening resembles the wavefront shown below