Question

what is fresnel's distance? obtain the equation for fresnel's distance
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Answer 1

$\huge\fcolorbox{black}{lime}{❥ᴀ᭄ɴsᴡᴇʀ}$

z ≈ (a2/ λ). This distance is known as Fresnel distance. Fresnel distance describes the distance at which spread due to diffraction becomes comparable to the width of the slit or not.