ok it is a tube long about 100 cm i want to make a laser beam enter the tube in an angle and reflect severl times in this tube like the black body....were the laser enter the body and reflect in it until it is complitly absorbed ...but instead of rectangel with one entrance ...a tube with two entrance
So you need to know how well the material absorbs the laser light.
At each reflection, some is absorbed, some transmitted, and some reflected.
Depending on the tube material and the color of the laser, the absorption could be 100% at the first contact to almost nothing. iirc: it does not depend on the angle of the incident beam so you are only concerned to get enough reflections to absorb a significant percentage of the photons in the length of the tube.
Lets say you want to reflect N times, in a tube length L and diameter D.
Put your laser on-axis, but angled towards one side.
If N=1, you need to aim the laser so it just grazes the edge of the entrance and hits half way down. If N=2, it's the same but aimed a third of the way down and so on. So you are making N+1 isosceles triangles, with height D and base 2L/(N+1). The angle of attack is therefore given by:
[itex]\tan\theta = L/(N+1)D[/itex]
If you aim the beam well off-axis though, you can get more reflections spiraling around the outside. So you have quite a lot of freedom... the same sort of argument above can be used to find out how many reflections you get off-axis.