question_answer

A slit, S is illuminated by a monochromatic source of light to give two coherent sources \[{{P}_{1}}\] and \[{{P}_{2}}\]. These give bright and dark bands on a screen. At a point R, on the screen, there is a dark fringe. What relationship must exist between the lengths \[{{P}_{1}}R\] and\[{{P}_{2}}R\]?                                                      

Answer:

                The condition for the dark fringe is \[p={{P}_{2}}R-{{P}_{1}}R=(2n-1)\frac{\lambda }{2},\]where \[n=1,2,3....\]