Explain by drawing a figure for the pattern of diffraction occurs by two slits.

Explanation

In figure, the actual double slit interference pattern the envelope shows the single slit diffraction.
In the double slit experiment the pattern on the screen is a superposition of single slit diffraction from each slit and the double slit interference pattern. This is shown in figure. Figure shows a broder diffraction peak in which there appear several fringes of smaller width due to double slit interference.
The number of interference fringes occurring in the broad diffraction peak depends on the ratio `(d)/(a)` that is the ratio of the distance between the two slits to the width of a slit where d is the distance between the two slits.
In the limit of a becoming very small the diffraction pattern will become very flat.
To obtain interference pattern, diffraction phenomenon does not need to happen but to obtain diffraction pattern, phenomenon of interference needs to happen.