Light is emitted from sources in an oscillating stream of discrete energy
packets, called photons. They are universally omnipresent and adhere to a unique physics system. The photon is the premier member of the classification of wave-particles, meaning it exhibits properties of both waves and particles. It does this by operating as a point of energy with zero mass and is so unique that a photon's speed helps dene how humans perceive time, space, and reality.
When illumination occurs, photons are present, being absorbed and reflected, and the ones reaching the eye provide sight. The human visual system processes photons in great numbers, not one at a time, so it's important to regard their greater behavior as a group.
Controvery Around Categorization
Light physics as an area of study has a long history. Definitive conclusions on what exactly light "is", took centuries and its history is full of theories that pushed other laws of the known universe. Theories of light began as controversial and sometimes blasphemous|scientic ideas, but in recent centuries physicists including Newton, Einstein, Young, and Planck built on each other's to form a complete idea of light.
The most controversial debate over light was whether it should be categorized as a wave or a particle. In the end, the categories were rewritten to allow for a photon to be both a wave and a particle.
The Wave System
To understand the debate surrounding the categorization of a photon as a wave or a particle one should consider the wave system. In physics, waves are oscillating repeating patterns. They can be mechanical and travel through a physical medium, much like the aptly named waves in water. There also exists an electromagnetic wave, meaning it travels through space, regardless of a physical medium being present. All waves exhibit the characteristic of being able to affect one another's patterns, known as interference.
The Particle System
Theoretical particles are multiple small amounts of mass or energy in which their group behavior is observable on a grander scale. When particles come together in a system one can ascribe additional macroscopic features such as density which refers to how close together the particles are packed.
These particles can group together or be broken apart and maintain a continuous momentum until some force acts upon them. Unlike waves, particle units do not phase through materials. When encountering an obstruction, particles must react by either absorption or reflection.
A wave-particle takes properties from both the behavior of waves and the behavior of particles and combines them into a new system.
Young's Double Slit Experiment
Consider Young's Double Slit Experiment, which proves that light
exhibits wave properties by showing that multiple photons can create interference patterns–a defining characteristic of waves. Young's Double Slit Experiment involves splitting a beam of light into two beams of light and then observing their resulting patterns on a surface. The light can be isolated using pinholes in a board or a more modern example involves splitting a laser into two beams by shining it through three pieces of lead. The two resulting beams then interfere with one another and this is evident by the rippled effect of the beams' outer edge when it hits a surface.
Photons also exhibit observable particle behavior; for example, shadows demonstrate the photons not phasing through materials but rather being absorbed like particles. Due to observable scenarios of light exhibiting both kinds of behavior, mainstream physics amended its previous notions. Einstein resolved the paradox surrounding the photon's classification with his famous E = mc2 equation stating that matter could transmute into energy. This new idea was paramount in the realization of the photon as a wave and a particle; even though the photon is the energy it could behave like a particle of matter in certain instances.
Light in Action
This addition to the theory of light was important because it explained how light can travel through the vacuum of space which is void of any medium. Mechanical waves, like sound waves, act upon matter and therefore they cannot travel through the vacuum of space. However light can travel through a medium-less void because it essentially acts as its own medium.
When light is emitted from a source, a chemical compound undergoes a
molecular change, shooting o extra energy in the form of a photon, leading to a stream of photons–the particles of the system. Photons are considered subatomic particles. The term 'subatomic' by definition means smaller than an atom and when visualizing a single photon interaction it is on this micro-scale.