(a) Consider a plane which is normal to the line joining two point coherent sources S1 and...

(a) Consider a plane which is normal to the line joining two point coherent sources S1 and S2 as shown in Fig. 14.12. If S1P – S2P = Δ, then show that

   where the last expression is valid for D >> d.

 (b) For λ = 0.5 μm, d = 0.4 mm and D = 20 cm; S1O – S2O = 800 λ. Calculate the value of S1P – S2P for the point P to be first dark ring and first bright ring.