**CBSE NOTES CLASS 12 PHYSICS **

**CHAPTER 10 WAVE OPTICS**

**Fringe width**

Dark and bright fringes are equally spaced and the distance between two consecutive bright fringes or two consecutive dark fringes is given by

$$\mathrm{f}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{}\mathrm{w}\mathrm{i}\mathrm{d}\mathrm{t}\mathrm{h},\mathrm{}\mathrm{\beta}=\mathrm{}{\mathrm{x}}_{\mathrm{n}+1}-{\mathrm{x}}_{\mathrm{n}}=\mathrm{}\frac{\mathrm{\lambda}\mathrm{D}}{\mathrm{d}}$$

The central point O will be bright because S_{1}O = S_{2}O and it will correspond to n = 0. If we consider the line perpendicular to the plane of the paper and passing through O [i.e., along the y-axis] then all points on this line will be equidistant from S_{1} and S_{2} and we will have a bright central fringe which is a straight line.

All other fringes will be hyperbolas. If D >> β then they will be almost straight lines.