text: re post of light thought experiment - this is a re post of the sequel to the question that i posted 2wks ago.? the scenario is a lighthouse at the north pole, whose beam rotates at 1 revolution per second. ignoring attenuation by the earth's atmosphere and the rotation of the earth, my original question was how far along the beam would it be travelling tangentially at the speed of light. the answer that were provided said that the "beam" is not solid and that in fact the photons in the beam are all travelling radially, with no tangential component. this i can accept. now for my next installment: imagine that there is a mirror in the shape of an anulus (ring) at a distance "d" from the lighthouse:  1) what is the value of "d" (let's call it d) such that the intersection of the beam of light and the anulus moves at the speed of light (c)? 2) if the anulus was at a distance of 2d from the lighthouse, what will the speed of the intersection of the beam and the anulus be? let's call it z. 3) what will i observe as i look towards the anulus from the lighthouse?(i.e. direction of reflected light you just executed my brain...