Diffraction occurs when the width of the narrow slit D-A is comparable to the wavelength of light used. In this case, the diffracted waves spread out and interfere with each other, resulting in a pattern of alternating bright and dark regions called a diffraction pattern. This phenomenon is known as single-slit diffraction.
When the width of the slit is much greater than the wavelength of light used, diffraction effects become negligible, and the light behaves more like a straight beam, without significant bending or spreading.
On the other hand, when the width of the slit is very small compared to the wavelength of light, the light is strongly diffracted, and the resulting pattern can exhibit complex interference effects, such as multiple bright and dark fringes. This is known as the regime of Fraunhofer diffraction, and it is commonly observed in experiments involving slits, gratings, or other aperture structures.