For light to qualify as a laser, it needs to meet two main criteria: coherence and amplification.
Coherence: Laser light is coherent, which means that the light waves are in phase with each other. In other words, the peaks and troughs of the light waves are synchronized, resulting in a well-defined and concentrated beam of light. Coherence allows laser light to be focused into a narrow beam that can travel over long distances without significant spreading.
Amplification: Laser light is amplified, meaning that the light waves are intensified or "amplified" through a process called stimulated emission. This amplification is achieved by using a gain medium, which can be a solid, liquid, gas, or semiconductor, among others. The gain medium is "pumped" with energy, which excites the atoms or molecules within it. When these excited particles return to their lower energy state, they emit light. This emitted light can then stimulate other excited particles to emit light with the same wavelength and phase, resulting in the coherent beam of laser light.
While a laser typically emits light in a focused beam, it's important to note that the defining characteristic of a laser is not the presence of a beam itself, but rather the coherence and amplification of the light. Laser light can be shaped and manipulated using various optical components to produce different beam shapes or patterns. This flexibility allows lasers to be used in a wide range of applications, including cutting, welding, medical procedures, scientific research, and communication systems.