State two conditions for sustained interference of light. Also write the expression for the fringe width.

Hint: The process of overlapping of two waves of same frequency and same wavelength and constant phase difference is called interference. Fringes will be formed on the screen due to interference and the intensity of fringes will be different at different points on the screen.

Complete step-by-step answer:
During interference of light, there will be a source which continuously emits light and at some distance from the source there will be two slits at some distance. The light from these slits will fall on the screen which is at distance(D) from the slits. The rays from the slits will have the same wavelength and frequency.
So two main conditions which are required for the sustained interference of light are
1) light rays from the slits must have constant phase difference and same frequency
2) light rays coming from slits must be continuous.
The other preferable conditions are that it is preferred to have the same wavelength for two light rays and the distance between the slits must be small.
The expression for fringe width will be
$\beta = \dfrac{{\lambda D}}{d}$
Where $\lambda $ is the wavelength and $D$ is the distance between slits and the screen and $d$ is the distance between the slits.
In order to observe the interference, the width of the slits must be small and distance between the slits must be very small.

Note: Fringe width is the distance between two successive maxima or two successive minima on the screen. Maximum is a place where bright fringe is formed and intensity is highest, minima is the place where dark fringe is formed and intensity is the lowest. At the central position of the screen, a bright fringe is formed.