Final answer:
The invisible barrier that separates the 1st and 2nd layers refers to different forms of barriers depending on the context, including structural sequences in crystallography, charge separation by the Coulomb force in electrochemistry, phase changes in thin-film interference in optics, and potential barriers that affect quantum tunneling in quantum mechanics.
Step-by-step explanation:
The invisible barrier that separates the 1st and 2nd layers in various contexts can be attributed to several phenomena. In the context of crystallography and solid-state physics, the barrier is not explicitly defined, but concerns can arise around stacking sequences in crystalline substances, where the layers are defined by the positions of atoms and how additional layers are added, resulting in repetitive sequences like A-B-A-B...
In electrochemistry, the invisible barrier refers to the formation of a charge separation across a membrane which is due to the Coulomb force, preventing ions from freely diffusing across the membrane. This creates a voltage across the membrane because of a separation of charge.
In optics, thin-film interference creates an invisible barrier of a phase change between two layers, such as the air gap between microscope slides. This affects light transmission, leading to constructive interference patterns like bright and dark bands or rainbow colors, depending on the thickness of the layer and the type of light used.
Quantum mechanics has it's own invisible barrier, known as a 'potential barrier', which can affect the movement of quantum particles. Particles can sometimes penetrate and tunnel through these barriers, a phenomenon explained by the physics of tunneling, depending on factors such as the energy of the particle and the width and height of the barrier.