Let's say you have 2 100% efficient reflective surfaces exactly parallel to each other. If you theoretically managed to place photons at exactly 90° angle of incidence. So this beam of visible light (and I want to stress that) is going to stay there as long as there is no interaction, the beam will stay there, right? What I want to know is if you could see this beam. If it exactly perpendicular to the reflective surfaces, and the light is only travelling on the sagittal plane relative to you, would you even be able to see it? Or could you see right through it?