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At its most fundamental level, light can be understood as a form of electromagnetic radiation that is visible to the human eye. Light waves are made up of oscillating electric and magnetic fields that travel through space at the constant speed of approximately 299,792 kilometers per second, a speed commonly referred to as the speed of light. These waves are characterized by their wavelength and frequency, which determine the color of the light.

For instance, violet light has a shorter wavelength and higher frequency, while red light has a longer wavelength and a lower frequency. Sunlight contains a broad spectrum of wavelengths ranging from violet to red, which is why we perceive it as white light. On the contrary, a coherent light source, like a laser, emits light with a very narrow range of wavelengths, often resulting in a single, pure color.

In incoherent light, the waves have random phases, meaning they do not match up consistently in time or space. Imagine a bustling crowd of people walking in different directions with no coordination; this is akin to how incoherent light behaves. Because of this randomness, incoherent light exhibits a variety of wavelengths and frequencies, all jostling independently. This quality leads to a more complex and varying pattern of light.

In everyday life, most light sources are incoherent, with sunlight being the most commonly experienced example. Sunlight is a mixture of all visible wavelengths, and this mix is why sunlight can illuminate objects with all the colors we perceive in nature. Furthermore, this type of light is typically emitted by sources such as incandescent bulbs and the sun, which radiate energy in a multitude of directions, resulting in a diffuse pattern of light.

Laser physics delves into the principles and mechanisms behind the generation and manipulation of coherent light. A laser, an acronym for Light Amplification by Stimulated Emission of Radiation, operates on the quantum mechanical principle of stimulated emission. This process involves exciting atoms to a higher energy level and then using a photon to stimulate the atoms to release additional photons of the same energy, resulting in a stream of identical light particles, or photons.

The heart of a laser is the gain medium, which can be a gas, liquid, or solid. This medium is where the stimulated emission takes place. Surrounding the gain medium are mirrors that reflect the light back and forth, amplifying it with each pass. One of the mirrors is partially transparent so that some light escapes as the coherent laser beam we can see or use. As a result, lasers emit light that is highly directional, monochromatic (single wavelength), and coherent, which is ideal for applications ranging from cutting and welding to scanning and surgery.