interference - Wave diffraction explanation

I'm trying to understand wave diffraction and I found this wikipedia article. It's in Czech so I'll explain a bit. I'm interested in the 4 images I couldn't find on english wikipedia. The first one is diffraction at large slit, second on large obstacle, third on tiny slit and last on slit which size is comparable with wavelength - happens with light on diffraction grating.

I got few question to those however. I don't know why, but I think I heard that wave can't pass through slit, which size is smaller than wavelength. For example imagine microwave oven. On doors is some kind of texture with those small slits. Microwaves have wavelength from 1 to 0.001 metres, so those slits should be sufficient and block the wave - that's why they are there in first place.

But how is then possible that third case with tiny slit?

Also, I'm missing cases what happens at obstacle, which size is comparable to wavelength and if I'm wrong in the above situation, also on a tiny obstacle? (And if I'm right with that microwave thing, does the wave pass as there was no obstacle when hitting something tiny? ).

On a slit, I can use Huygens principle to create envelope, but what to do on obstacles?

And can someone tell me what range of sizes compared to wavelength is considered for comparable obstacle?

And if it matters I imagine waves as always coming from source point, the way Kirchhoff described them as shown here, but I don't need to understand those equations and so on. I'd also appreciate if someone got a good website, images, documents,... well whatever that tryes to explain diffraction without too many formulas. I'm not going to compute anything in the end, just trying to get the best possible image of it all for now.

Answer

I don't know why, but I think I heard that wave can't pass through slit, which size is smaller than wavelength.

This is simply wrong. The energy passing through a slit/hole smaller than wavelength is less than transmitted through an comparable area in free field,
but it is not zero!

The real world, being wavelike and particle-like at the same time is not on/off, black/white, yes/no, true/untrue. Microwaves from Your oven are damped at that grating, but not erased totally. (BTW, my MW oven operates at about 12 cm wavelength, not ""1 to 0.001 metres,"" blocking the waves were almost impossible then)

On a slit, I can use Huygens principle to create envelope, but what to do on obstacles?

You could use Huygens principle on both edges of the obstacle, going outward for some length of say, two to three obstacle diameters, or read something about complementarity of slits and obstacles.