question_answer

The source is at some distance from an obstacle. Distance between obstacle and the point of observation is \[b\] and wavelength of light is\[\lambda \]. Then the average distance of nth Fresnel zone will be at a distance ... from the point of observation.

A) \[\frac{bn\,\lambda }{2}\]                                          

B) \[b-\frac{n\,\lambda }{2}\]

C) \[b+\frac{n\,\lambda }{2}\]                        

D) \[b-n\,\lambda \]