What is Fresnel distance? derive an expression for it.
Fresnel distance: It is the minimum distance that a beam of light has to travel before its deviation from straight line path.
From the theory of diffraction of light for a single slit, the angle of diffraction for central maxima is
sinθ ≈ θ = λ/a
After travelling distance x, the half angular width of beam would become (xλ/a).
This beam width will be more than width of slit(a).
So, (xλ/a) ≥ a
=> x ≥ (a2/λ)
Here, (a2/λ) is called Fresnel distance.
In optics and radio com munications (indeed, in any situation involving the radiation of waves, which includes electrodynamics, acoustics, and gravitational radiation), a Fresnel zone, named for physicist Augustin -Jean Fresnel, is one of a (theoretically infinite) number of concentric ellipsoidswhich define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture.