# Is Fresnel diffraction really visible if slit width is similar to distance from screen or does diffraction disappear in that case?

Diffraction phenomena occur when, named $a$ the amplitude of a slit and $\lambda$ the wavelenght, $a \sim \lambda$ or even $a<\lambda$ (full illuminated screen).

Therefore if $a\gg\lambda$ diffraction phenomena are not visible.

On the other hand there are two types of diffraction: Frahunhofer diffraction and Fresnel diffraction.

In the first case the assumption is that, named $L$ the distance between the slit and the screen and $L'$ the distance between the source and the slit, $L'\gg a$ and $L\gg a$ (the wavefronts are planes).

Fresnel is the opposite case, in particular $L\sim a$. But initially I stated that diffraction phenomena are not visible if $a\gg \lambda$. Therefore, assuming (reasonably) that $L\gg \lambda$ how can Fresnel diffraction phenomena occur at all.

In other words, If I have $L \sim a$ or even $a>L$ but still $L\gg \lambda$ do I see diffraction (Fresnel diffraction) or do I just see one light spot (no diffraction)?