Light is arguably one of the weirdest topics a physics student will encounter. The fastest thing in the universe is somehow both a particle and a wave – and exhibits the unique properties of both at the same time. But what ​is​ light?

Understanding what ​photons​ are and what ​quantization​ means is fundamental to understanding the nature of light, quantum physics and myriad related phenomena.

Photons are the formal name for light particles. They can be visible to humans or not, since here the term ​light​ is used in the physics sense, which means a photon is a particle of electromagnetic radiation at any frequency on the spectrum, from radio waves to gamma rays.

Photons are a ​quantized​ particle. This means they exist only in discrete amounts of energy, rather than any amount of energy in between. When considering the more chemistry-oriented description of a photon as the energy released when an electron falls to a lower energy level in the atom, this makes sense: Electrons can only be in specific orbitals, or energy levels. There are no half-steps. So if a photon is the result of a "falling electron," a photon must also come in only specific energy amounts, or quanta.

Albert Einstein introduced the notion of light quanta (photons) in a 1905 paper. One of four papers he published that year that revolutionized science, this was the idea that won him the Nobel Prize.

As mentioned earlier, light refers to any type of electromagnetic radiation, the types of which are distinguished by their differing frequencies (or wavelengths). Those two measures being characteristics of waves, it follows that light must be an ​electromagnetic wave.​

But wait – in the previous section of the article light was introduced as a ​particle​, the photon, not as a wave. This is correct. The weird nature of light is to exist in what is called wave-particle duality: ​It is both a wave and a particle.​

Therefore both "electromagnetic wave" and "photon" are acceptable descriptors of light. Usually the first phrase is used to describe light when it is ​acting as a wave​ and the latter term when it is ​acting as a particle​.

This becomes important depending on the phenomena a physicist is examining. In certain situations and in certain experiments, photons act like physicists expect particles to act, for instance, when observing the photoelectric effect. In other situations and experiments, light acts more like waves, such as when modulating a radio station.

Anything restricted to discrete values rather than existing on a continuous spectrum is undergoing quantization.

Quantization in an atom explains that the amount of energy that can be emitted in the form of a photon will occur only in multiples of the elementary unit Planck's constant, ​h​ = 6.6262 x 10 ^-34 joule-seconds

This unit, discovered by Max Planck in the late 1800s, is one of the most bizarre and important units in physics. It describes the relationship between a wave-particle's frequency and its energy level, and thus sets a lower lower limit on the certainty with which we can understand the structure of matter.

One of the biggest ramifications of knowing this limit, which also helped begin the odd-but-real field of study known as quantum physics, is that at the smallest sub-atomic levels the position of particles is only describable as a probability. Put another way, only a sub-atomic particle's position ​or​ velocity can be known with certainty at any given time, but ​not both​.

Defining the quanta ​h​ lead to an equation for the energy of a photon:

where energy ​E​ is in joules (J), Planck's constant ​h​ is in joule-seconds (Js) and frequency ​f​ is in hertz (Hz).

Most people probably think of particles as being tiny units of matter, which are sized according to their masses. This makes the particle form of light a particularly strange beast since, as a unit of pure energy, a photon has zero mass.

Another important property of photons is that they always travel at the speed of light, ~ 300,000,000 m/s in the vacuum of empty space. Light can travel slower than that – any time it encounters other matter it interacts with it and slows down, so that the denser the material through which light is traveling, the slower it goes. However, ​nothing in the universe can travel faster than light​. Not the fastest rocket nor the most accelerated atomic particle.

In this way, understanding light is critical to understanding the fundamental limits of the universe itself, from its very largest to its very smallest.

Although light always travels at the same ​speed​ in a given medium, as a form of electromagnetic radiation, it can have different ​frequencies​ or ​wavelengths​. The frequencies and wavelengths of light as electromagnetic waves change inversely with one another along a spectrum.

At the longest wavelength and lowest frequency end are radio waves, after which come microwave, infrared, visible light, ultraviolet, X-ray and high-energy gamma rays, each with progressively shorter wavelengths and higher frequencies.

Physicists in the 1930s began to learn that all the matter in the universe is composed of a few fundamental particles, known as elementary particles, which are all governed by the same set of fundamental forces. The ​Standard Model​ of particle physics is a set of equations attempting to succinctly describe how all of these elementary particles and the fundamental forces relate. Light is a critical piece of this universal description.

In development since the 1970s, the Standard Model has so far correctly predicted the results of many, though not all, quantum physics experiments. A glaring problem yet to be resolved in the model is how to incorporate gravity into the set of equations. Additionally, it fails to provide answers about some large cosmological questions, including figuring out what dark matter is or to where did all the antimatter created in the Big Bang disappear. Still, it is widely accepted and considered the best theory for explaining the fundamental nature of our existence to date.

In the Standard Model, all matter is made up of a class of elementary particles called ​fermions​. Fermions come in two types: ​quarks​ or ​leptons​. Each of these categories is further divided into six particles, related in pairs known as ​generations​. The first generation is the most stable, with heavier and less stable particles found in the second and third generations.

The other components of the standard model are forces and carrier particles, known as ​bosons​. Each of the four fundamental forces – gravity, electromagnetic, strong and weak – is associated with a boson that conveys the force in exchanges with matter particles.

Particle physicists working at accelerators or watching for high-energy particle collisions from space have identified bosons for the latter three forces. ​The photon is the boson that carries the electromagnetic force in the universe​, the ​gluon​ caries the strong force and the ​W​ and ​Z​ particles carry the weak force. But the theoretical boson for gravity, the ​graviton​, remains elusive.

​Blackbody radiation.​ Blackbodies are a hypothetical type of object (perfect ones do not exist in nature) that absorb all of the electromagnetic radiation that hits them. In essence, any electromagnetic radiation hitting a blackbody serves to heat it and the radiation that it gives off while cooling is therefore directly related to its temperature. Physicists can use this approximation to deduce the properties of near-perfect blackbodies in the universe, such as stars and black holes.

While the wave nature of light helps describe the frequencies of blackbody radiation that an object will absorb and emit, its particle nature as a photon also helps describe it mathematically, since the energies the blackbody can contain are quantized. Max Planck was among the first to investigate this phenomena.

​The double-slit experiment.​ A central tenet of quantum physics, the double-slit experiment shows how shining a light on a barrier with two narrow openings results in a distinctive pattern of light and dark shadows known as a ​wave interference pattern​.

The strange part of this is that a single photon shown through the opening will still behave as if it is interfering with other photons, despite being alone and indivisible. This is to say that the light pattern observed in the experiment cannot be explained by treating light as only a photon or a wave; it must be considered both. This experiment is often cited in explaining what is meant by the idea of wave-particle duality.

​The Compton effect.​ The Compton effect is another observable example of the interplay between light's wave and particle natures. It describes how both energy and momentum are conserved when a photon collides with a stationary electron. Combining the equation for the amount of energy of a photon with momentum conservation equations shows that the resulting wavelength of the outgoing photon (the initially still electron) can be predicted by the wavelength of the incoming photon that gave it energy.

​Spectroscopy.​ The technique of spectroscopy allows physicists, chemists, astronomers and other scientists to investigate the material makeup of an object, including distant stars, simply by analyzing the patterns that result from splitting incoming light from that object with a prism. Because different elements absorb and emit photons in discrete quanta, the observed electromagnetic wavelengths fall into discrete segments depending on what elements the objects contain.

​Mass-energy equivalence.​ Plenty of children can recite Einstein's famous equation ​E = mc²​. Short and sweet, the true implications of this equation are profound: ​Mass ​m​ and energy ​E​ are equivalent​ and can be converted to one another using the speed of light in a vacuum, ​c​, squared. This importantly implies that an object that is not moving still has energy; in this case its ​rest mass​ is said to equal its ​rest energy​.

Particle physicists use mass-energy equivalence to determine simpler units for some of their measurements. For example, quantum physicists search for the masses of fermions or bosons by accelerating subatomic particles like protons and electrons to near-light speeds in giant accelerators and smashing them together, and then analyzing the effects of the "debris" in highly sensitive electrical arrays.

Instead of giving a mass in kilograms, however, the common way of reporting particle masses is in giga-electron-volts, or GeV, a unit of energy. To return this value to a mass in the SI unit of kilograms, they can use this simple relationship: 1 GeV/​c​² = 1.78266192×10⁻²⁷ k.