Fresnel distance is defined as the minimum distance that is travelled by a ray of light along the linear path before diffraction.
Derivation of Fresnel distance
Consider a single slit of an aperture with “a” as the thickness of the aperture. A diffraction pattern is observed during the experiment. A central maxima is observed due to diffraction.
Angular size of the central maxima = \(frac{lambda }{a}\)
Let z be the distance travelled by the light waves so that the spread occurs
The spread is given as = \(frac{zlambda }{a}\)
Where,
\(frac{zlambda }{a}\) is the width of the diffracted beamTherefore,
Fresnel distance is given as \(zapprox frac{a^{2}}{lambda }\)
