Diffraction through a single-slit and around objects

∙If a wave meets a hole in a wall that is of comparable size to its wavelength, the wave will be bent through a process called diffraction.

∙If the aperture (hole, opening, etc.) is much larger than the wavelength, diffraction will be minimal to nonexistent.

FYI ∙

Diffraction is caused by objects within the medium that interact with the wave. It is not caused by two mediums and their boundary.

∙Christian Huygens explained the behavior of diffraction through his famous principle:
“Every point on a wavefront emits a spherical wavelet of the same velocity and wavelength as the original wave.”
∙Note that it is because of Huygen’s principle the waves can turn corners.
The reason waves can turn corners is that the incoming wave transmits a disturbance by causing the medium to vibrate.
∙And wherever the medium vibrates it becomes the center of a new wave front as illustrated to the right.

∙Note that the smaller the aperture b the more pronounced the diffraction effect.

FYI ∙

The aperture size must be of the order of a wavelength in order for diffraction to occur.

Path difference

∙This animation showing two coherent (in-phase and same frequency) wave sources S₁ and S₂ will show the following:

 
∙This animation showing two coherent wave sources S₁ and S₂ will show the following:

Diffraction through a single-slit

∙Huygen’s wavelets not only allow the wave to turn corners, they also interfere with each other.

EXAMPLE: If light is diffracted by a circular hole, a planar cross-section of the interference looks like the picture below. What will the light look like head-on?
SOLUTION:
∙Picture the waveform of the previous slide in 2D