A laser creates a beam of light that is more coherent than the sun's rays due to a process called stimulated emission and the design of the laser cavity.
In a laser, the coherence of the light is a result of the synchronized emission of photons from a large number of atoms or molecules. Here's a simplified explanation of how it works:
Stimulated Emission: The laser medium, which can be a solid, liquid, or gas, contains atoms or molecules in an excited state. When a photon of the right energy interacts with an excited atom, it stimulates the atom to release a second photon that is identical to the stimulating photon. This process is called stimulated emission, and it leads to the amplification of the light.
Population Inversion: To achieve a significant amount of stimulated emission, a population inversion is created in the laser medium. Normally, at thermal equilibrium, more atoms or molecules reside in the lower energy state (ground state) than in the higher energy state (excited state). However, in a laser, the atoms or molecules are pumped into the excited state, creating a population inversion where more atoms or molecules are in the excited state than in the ground state.
Optical Feedback: The laser cavity is designed to provide optical feedback, which is crucial for coherence. It consists of two mirrors, one fully reflective and the other partially reflective. The partially reflective mirror allows a small portion of the light to escape and form the laser beam. When a photon travels back and forth between the mirrors, it encounters other excited atoms or molecules, stimulating further emission and amplification. This process, combined with the population inversion, leads to the buildup of coherent light.
By maintaining a population inversion and using optical feedback within the laser cavity, the laser produces a highly coherent and directional beam of light. The coherence arises from the fact that the photons emitted from the excited atoms or molecules have the same energy, phase, and direction. In contrast, sunlight is a result of incoherent emission from a vast number of atoms and molecules in the Sun's hot plasma, leading to a broad spectrum of wavelengths and a lack of phase coherence.
The coherence of a laser beam allows it to propagate over long distances with minimal spreading or divergence, making it well-suited for applications such as laser pointers, laser communication, and precision measurements.