The diffraction pattern is observed when the size of the object is comparable to the wavelength of the diffracting light.

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. This bright spot is a central maxima observed in the case of diffraction from the single slit. This is because light waves are diffracted from the edge of the tiny circular obstacle, which interferes constructively at the centre of the shadow. This constructive interference produces a bright spot.