What is defined as the states of equilibrium or middle state of light?

The term that is defined as the states of equilibrium or middle state of light is the metastable state. In the context of atomic and molecular physics, a metastable state refers to an excited state of a system that has a relatively long lifetime compared to other excited states. It is not the most stable state (which would be the ground state) but is also not in a state of rapid transition to other configurations.

In a metastable state, the energy of light is maintained in equilibrium, allowing for the possibility of various processes such as stimulated emission to occur, especially in lasers. This is crucial for understanding how laser technology functions, as photons can be emitted from the metastable state under specific conditions, contributing to the amplification of light which is fundamental in laser operation.

The other choices like ground state and ground level refer to the lowest energy states of atoms or molecules, while equilibrium state is a broader term that doesn't specifically capture the unique characteristics of a metastable state in the context of light.