# CBSE Class 12-science Answered

**what is the formula for fringe width in a diffraction pattern? how does it vary for minimum ,maximum? and also how is it different from the formula for fringe width in interference pattern?**

- Diffraction at a single slit (Fraunhofer diffraction)

The phenomenon of bending of light round the sharp corners of an obstacle and spreading into the regions of the geometrical shadow is called diffraction.

**Expression For Fringe Width**

Consider a parallel beam of light from a lens falling on a slit AB. As diffraction occurs, the pattern is focused on the screen XY with the help of lens L_{2}. We will obtain a diffraction pattern that is a central maximum at the centre O flanked by a number of dark and bright fringes called secondary maxima and minima.

**Central Maximum** ? Each point on the plane wave front AB sends out secondary wavelets in all directions. The waves from points equidistant from the centre C lying on the upper and lower half reach point O with zero path difference and hence, reinforce each other producing maximum intensity at point O.

**Positions and Widths of Secondary Maxima and Minima**

Consider a point P on the screen at which wavelets travelling in a direction making angle *?* with CO are brought to focus by the lens. The wavelets from points A and B will have a path difference equal to BN.

From the right-angled ?ANB, we have

BN = AB sin*?*

BN = *a* sin*?*
(i)

Suppose BN = *?* and *? = ?*_{1}

Then, the above equation gives

? = *a* sin *?*_{1}

Such a point on the screen will be the position of first secondary minimum.

If BN = 2*?* and *?*= *?*_{2}, then

2*?* = a sin *?*_{2}

Such a point on the screen will be the position of second secondary minimum.

The fringe width is given by

? = ?D/d

?=fringe width, D= distance between slits and screen , d=distace between slits and also , ?= wavelength