We know sources of energy makes waves that emanate in a circular/spherical manner.
Occurs when waves either pass through a gap or around an obstacle and the wavefronts are deformed (bent, their shape changes), but ONLY IF $\lambda\geq d$ where $\lambda$ = wavelength, $d$ = size of gap.
Remember: wavelength is not the length of the wavefront lines! It's the distance between wavefronts.
