modelling an optics experiment

Hi galwiner85, I wouldn't use a Curve or Path for this; that will most likely not give the desired result due to the sharp reflections you need to get at the mirrors.

It can be done with a cylinder, but don't use a lot of vertices (something like 8, maybe, you can use a Subdivision Surface Modifier later if you need the beam to be more round).

Now, I don't know how well you know Blender already, but mainly there are 3 things you'll need for this (at least if you want to do it 'my way'):

If you select the mirror that you are working on, press Shift Numpad 1. (Or View > Align View > Align View to Active > Front.)

In your Transform Orientation, change that from Global to View.

Make sure that your beam goes through the mirror.

Use the Knife Tool to cut the beam where it passes through the mirror, constraining it and cutting through the mesh (K C Z).

Delete the Vertices that are on the wrong side of the mirror.

Now,  select your beam, go into Edit Mode, duplicate the last straight beam stretch and duplicate it.

Make sure that your Pivot point is set to Median Point. Select the edge ring that touches the mirror and set the Cursor to the selection. (Shift S > Cursor to selected.)

Now select the duplicated beam and mirror it along the X-axis (S X -1).

Again get out your Knife and cut though the middle, perpendicular to the mirror (using constraint and cut through, like before).

Remove Double Vertices (in 2.80 that is under Merge (Alt M) > Merge by Distance) and delete anything that shouldn't be there (like inner Faces and so on).

Now repeat this process for every mirror andyou'll end up with something like this (hopefully):

Maybe someone else knows an easier way, but this is how I would do it;)

I hope this is helpful. It's not so easy to try and describe this process...