About diffraction (conceptual problem)

[imvaibhav]

I have read that the wavelength must be according to the size of aperture to get diffraction phenomena. Considering normal incidence on the aperture,
please explain why the wavelength should be of the order of aperture, as wavelength is perpendicular to the diameter of aperture, & any wave can go through the aperture.
(Sorry if its a noobish question)

 

[imvaibhav]

No reply ;-(

 

[Andy Resnick]

Diffraction effects will occur for any size aperture- the effects become dominant when the aperture size is on the order of the wavelength.

Your question is quite perceptive tho- you are correct, the wavelength is associated with momentum directed along the direction of propagation, essentially normal to the aperture. Perhaps the most simple explanation is that as the aperture shrinks, 'essentially normal' is no longer valid, and the transverse momentum becomes more and more important.