I was watching a video of Sabine Hossenfelder, in which interferometers were a topic of discussion. Excuse me if the answer is obvious because I am not a physics pro by any means, just an interested reader.
She mentioned following construction:
Basically, a laser is getting shot at a beam splitter, which transmits and reflects 1/2 of the beam, respectively. Then the beams are getting reflected by a mirror on each side until they meet at the next beam splitter. Then both beams will recombine to the original beam which is measured by the detector D2.
However, my question is: Why does the first beam splitter reflect and transmit 1/2 of the beam respectively, while the second one does not? The beam on the bottom is only transmitted, the one on the right only reflected so that the whole original beam intensity is detected at D2.
She mentioned that is has something to do that the path of the two beams is the same length, but what is the underlying physical law here that it works like this and how is it calculated?
It is also mentioned that the second beam splitter indeed reflects and transmits if the path length for both is not the same. Why?