How Does Reflection Cause a Phase Change?

The two photons that could travel towards detecto two, soy detecta one, cancel each other out. They destructively interfere but they constructively interfere. Why should that be? Well, if you know a little bit of physics, reflection from mirrors cause a phase change. As it's reflected off, it changes from being up to being down. And so it's up as it goes through here. But when a photon passes through glass first and then hits the rear of the beam splitter, causing reflection up to detect one, the phase change is different. It doesn't happen there.