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)  \[b-n\lambda \]