Final answer:
A population inversion and metastable upper level are essential for laser action to occur. Lasers function through stimulated emission when more atoms are in an excited state than in the ground state, and the laser medium does not have to be a gas. Option b is the correct answer.
Step-by-step explanation:
To address the question regarding the essential condition for laser action to occur, we need to consider the following options:
The lower level is the ground state of the atom.
A population inversion exists.
The upper level is metastable.
The laser medium is a gas.
All of the above are essential.
Laser action requires certain conditions to be met in the material that is used to produce the laser light. Specifically, a population inversion is necessary for laser action, which means that more atoms or molecules are in an excited state than in the ground state. This is crucial because it allows the process of stimulated emission to proceed more readily than absorption, leading to an amplification of light. Furthermore, for a laser to function efficiently, the excited state, or upper level, needs to be a metastable state. A metastable state is one in which an electron remains excited for a relatively long period, which increases the probability of stimulated emission.
Although lasers can be constructed from various types of materials including gases, liquids, solids, and semiconductors, it is not essential for the laser medium to be a gas. Therefore, given the options provided, the correct answer is that a population inversion exists and the upper level is metastable are essential for laser action. Option b (A population inversion exists) is the correct option to choose.