Slit width order of wavelength

[jd12345]

In diffraction why the slit width has to be the order of wavelength of wave? I don't understand. The smaller the width of the slit i think the more effective diffraction becomes.
Why the width has to be order of the wavelength?

 

[Claude Bile]

It doesn't. It can be much smaller or much larger. Diffraction theory can easily account for both.

Look up "Kirchoff integral theorem".

Claude.