What is light, and in what ways does a laser differ from a flashlight? -PART IIHave you ever contemplated the differences between a laser and a conventional flashlight? Addressing this inquiry necessitates a succinct historical overview and a detailed examination of the nature of light. This will be the focus of our discussion now.
What is the relevance of the laser in this context?
This raises the question: Is it even feasible to produce a light source capable of generating the purest possible electromagnetic waves across any frequency spectrum? The answer is the laser. More precisely, the answer resides in the principle of stimulated emission of photons, which underpins its operation.
All the light sources described heretofore emit photons passively, as a consequence of other processes. However, Einstein himself proposed as early as 1916 that electrons could be induced to emit photons collectively simply by prompting their transition from one orbital level to another.
For this phenomenon to occur consistently, it is essential that the majority of atoms within a given volume be in an excited state. Additionally, the environment in which the process occurs must be enclosed by a mirrored surface capable of reflecting photons. As a result, the number of photons within the generator will continuously increase, and it is then sufficient to leave a single narrow aperture through which they can pass, thereby producing a beam of intense radiation.
A distinguishing characteristic of such a system is that, akin to LEDs, all emitted photons possess identical frequencies. Nevertheless, a notable distinction exists between them. Spontaneous emission in semiconductors transpires with phase and directional mismatches. Conversely, in a laser beam, all electromagnetic oscillations are flawlessly superimposed. This phenomenon is termed coherence, and it fundamentally differentiates a laser from all other sources of light.
Lasers emitting at various wavelengths. Source: Wikipedia.
Recall the initial stages of our study on light? It began with rays. However, rays emanating from all light sources gradually diverge in every direction as they propagate, and at a certain distance, they form a large spot. In contrast, with lasers, due to their coherence, this process occurs at a significantly slower rate.
This is precisely why they are utilized as indicators. The concept underlying beam weapons similarly relies on this principle, as well as on the fact that pumping a medium can produce a significantly large number of photons. Furthermore, a high photon density per unit cross-sectional area of the beam corresponds to a substantial amount of energy being transferred to the material.
In essence, the laser is regarded as the most exemplary source of illumination ever developed by mankind. It is therefore not surprising that the United Nations designated the date of its initial demonstration as World Light Day. After all, it was at that juncture that humanity achieved comprehensive mastery over this enigmatic phenomenon, the nature of which had been subject to debate for centuries.
