# Main Features Of Laser

eq...(1)

where l = SS′, d is the distance between pinholes S and S2 and D is the distance of the pinholes from the sources S and S′. Equation (1) gives

Equivalently, for an extended source (of width l) the interference pattern of good contrast will be formed if the separation of the two point sources, d is

eq...2

Now l/D is the angle subtended by the source at the slit as shown in Fig. 3 i.e.,

fig..(3)

Substituting the value of l/D in Eqn. (2), we get

In practice the pinhole S_{1} is fixed and pinhole S_{2} is moved to see the reduction in fringe contrast. The area over which the pinhole S_{2} shows the interference fringes is called the ‘Coherent Area’ of light wave. The distance d_{t} between the pinholes for which the fringes just disappear is called transverse, lateral or spatial coherence length, Thus