# Explain The Term Fresnel Distance

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 beam

Therefore,

Fresnel distance is given as $zapprox frac{a^{2}}{lambda }$