What is the condition for diffraction from an obstacle?

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Diffraction of light.

What is diffraction of light?

The position of the direct image obtained at $O,$ when a monochromatic beam of light is passed through a plane transmission grating at normal incidence is shown in figure. The diffracted images $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ correspond to the first, second and third order diffraction. When the source is replaced by another source of shorter wavelength

In the figure below, $P$ and $Q$ are two equally intense coherent sources emitting radiation of wavelength $20\mathrm{m}.$ The separation between $\mathrm{P}$ and $\mathrm{Q}$ is $5\mathrm{m}$ and the phase of $\mathrm{P}$ is ahead of that of $\mathrm{Q}$ by $90°.\mathrm{A},\mathrm{B}$ and C are three distinct point of observation, each equidistant from the midpoint of PQ. The intensities of radiation at $\mathrm{A},\mathrm{B},\mathrm{C}$ will be in the ratio :

When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?