As said, these experiments are a step aside. They were carried out without enthusiasm. But, nevertheless, they gave food for thought.
The addition of light waves. This is indicated in physics textbooks. At one angle, the waves add up in antiphase, creating darkness. At a different angle, they reinforce each other. In fact, the question of where the light will be and where the darkness will be is decided earlier. The lattice itself. Which is supposed to have many wonderful properties.