Define diffraction. Deduce an expression for fringe width of the central
maxima of the diffraction pattern, produced by single slit illuminated with
monochromatic light source.​

Dear Student ,
The process by which a bean of light or a system of waves is spread out and as a result of passing through a narrow aperture or across an edge accompanied by interference between the wave forms produced is known as the Diffraction phenomena .

Now as we know that in case of a single slit the condition for minimum ,
$$\begin{gathered} \tan \theta =\frac{y}{D} \\ Again , \tan \theta \approx \sin \theta \approx \theta \approx \frac{y}{D} \\ Condition for minimum, \\ a \sin \theta =m\lambda \\ \Rightarrow y=\frac{m\lambda D}{a} \\ Now for the central maxima , fringe width , y =\frac{2\lambda D}{a} \end{gathered}$$
Here 
Regards