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Assume I have a thick wall made of material that is almost a black body. I have a very long hole in that wall (tunnel). The hole is very thin with diameter near $0.005 \ mm$, but diameter is still more than wave length of visible light. The inner surface of that hole is almost ideal, I mean there is no roughness. Then I start flashing the light throught that hole.

My question is :

Will the beam of light have cylidrical form after the wall or it will be scattered? If it will be scattered then how can I get nonscattered beam? I mean cone of light with little angle, that is not laser.

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If you want a cone of light the only way is to have a collimated beam, of a smaller wavelength, passing through the hole. In all other circumstances, you'll get a diffracting beam. Regarding the lens, you can not avoid diffraction with a lens after the tunnel exit, but you can try to collimate it...but it will probably be quite difficult

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