Diffraction of waves

Diffraction of waves is a phenomenon in which waves spread out as they pass through an aperture or round a small obstacle. The wave curve round the edges of a obstacle. The amount of curving depends on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the wavelength, the curving will be almost unnoticeable. However, if the two are closer in size or equal, the amount of curving is considerable, and easily seen with the naked eye.

The effect of diffraction is most obvious when λ ≥ a. This happens when:

  1. the size of the aperture or obstacle, a is small enough,

  2. the wavelength is large enough.

Characteristics of Diffraction of waves:
  1. The frequency, wavelength and speed of the waves remains the same after diffraction.
  2. The direction of propagation and the pattern of the waves change.
  3. The effect of diffraction is most obvious when λ ≥ a, a = size of the aperture.
  4. The amplitude of the part of the wave that spreads out is lower - as the energy is distributed over a bigger area.

Diffraction of water wave    

λ < a

Diffraction through a big aperture. The effect of diffraction is not obvious. The waves are bent only at the edges after passing the slit.

λ ≥  a

Diffraction of a small aperture (about the size of the wavelength of the water wave). The effect of diffraction is obvious. The waves are circular and appear to originate from the small slit.
   The 1st diagram on the left shows diffraction of water wave on a large obstacle. Compare to when it pass through a small obstacle. The effect is more obvious and the waves recombined after passing through the obstacle. Diffraction through a small aperture and a small obstacle. The wave is extended to the shadow region and curve at the edges.

  

 

a = gap size

video clip on diffraction of water wave.