# Photon

A photon is a particle that carries electromagnetic radiation. It is the fundamental unit of light.

##### Photons properties
• Photons, the elemental particles of electromagnetic radiation, possess distinct properties:
• Energy: The energy of a photon is contingent upon its frequency or wavelength. Higher frequencies correspond to greater energy levels.
• Number: Intensity or brightness dictates the number of photons present in electromagnetic radiation. Higher intensities correlate with larger photon counts.
• Direction and Polarization: Photons travel in straight paths, but interactions with matter can alter their direction. Additionally, photons can exhibit polarization, indicating the orientation of their electric and magnetic fields.
• Speed: Photons travel at the speed of light, an astounding 299,792,458 meters per second in a vacuum.
##### Photons and mass
•  The statement above about zero rest mass can be broken down as follows:
• According to the theory of relativity, any object that has mass needs energy to accelerate.
• The amount of energy required to accelerate an object increases as the object’s mass increases.
• Photons are unique in that they have zero rest mass and this means they do not require any energy to be accelerated.
• As a result, they always move at the speed of light in a vacuum.
• So photons always travel at approximately 299,792 kilometres per second and, when undisturbed, they never decelerate or come to a halt.
##### Energy of photons, wavelength, frequency and colour
• The energy of a photon (its photon energy) is intrinsically linked to its wavelength and frequency, and in perceptual terms, to its colour.
• Photon Energy: The energy of a photon, E, is given by the equation E=hf, where h is Planck’s constant and f is the frequency of the photon. So, a photon with a higher frequency has higher energy.
• Frequency and Wavelength: Frequency (f) and wavelength (λ) are related by the speed of light (c), through the equation c=fλ. So, photons with a higher frequency have a shorter wavelength, and photons with a lower frequency have a longer wavelength.
• Colour Perception: In terms of colour perception, different frequencies and wavelengths of electromagnetic energy are perceived as different colours.
• For example, photons with high energy (high frequency, short wavelength) are perceived as blue/violet, while photons with lower energy (low frequency, long wavelength) are perceived as red. The range of frequencies (or wavelengths) that human eyes can detect is known as the visible light spectrum.
##### Photons and interaction with charged particles
• Photons can interact with charged particles such as electrons within atoms. In such events, they can either be absorbed, resulting in the elevation of the particle to a higher energy state or be emitted when a particle transitions from a higher energy state to a lower one.
• A higher energy state refers to a quantum state or level of an atomic or subatomic system in which an electron or particle has absorbed energy and moved to a more excited or elevated position, typically farther from the nucleus or centre of the system.
##### Photons and interaction with matter
• Photons can engage with matter through various processes. They can be scattered, absorbed, or emitted during interactions with atoms and molecules. These processes are crucial for a range of phenomena from the heating of surfaces under sunlight to the transmission of information in fibre optic cables.
##### Photons and polarization
• Photons can be polarized. This means their electric and magnetic fields can oscillate in a specific orientation. Polarization is used in various applications, from LCD screens to polarized sunglasses, and is also a significant aspect of certain quantum mechanical phenomena.
##### Photons and their Energy, Frequency, Wavelength and Momentum
• The energy of a photon determines its frequency, wavelength and momentum. This energy can be transferred during interactions, leading to phenomena like fluorescence and the photoelectric effect. In the visible spectrum, different energies (and therefore frequencies and wavelengths) correspond to different colours of light.
• The energy of a photon is directly proportional to its frequency. This means that photons with higher frequencies have more energy. The energy of a photon is also inversely proportional to its wavelength. This means that photons with longer wavelengths have less energy. The momentum of a photon is inversely proportional to its wavelength. This means that photons with shorter wavelengths have more momentum.
##### Photons and wave-particle duality
• Wave-particle duality refers to the phenomenon where entities like light can exhibit characteristics of both waves and particles.
• Electromagnetic radiation, including light, is often described using wave properties. However, when it interacts with matter, it behaves like particles.
• A photon is a quantum of electromagnetic radiation and represents the smallest discrete amount of light energy.
• When a photon is absorbed by matter, the energy becomes localized at specific points. This phenomenon is termed ‘wave function collapse.’ It describes the transition of a quantum system from a superposition of states to a definite state upon measurement.
• Wave-particle duality is a fundamental aspect of quantum mechanics and applies to all particles, not just light. Particles like electrons also exhibit wave-like and particle-like behaviour.
• The double-slit experiment is an experiment in quantum physics that demonstrates the wave-like behaviour of particles, including photons and electrons, and is a key illustration of wave-particle duality.
###### References
• A photon is a particle that carries electromagnetic radiation. It is the fundamental unit of light.
• Thinking of photons as particles is useful for understanding the quantum nature of light.
• In the world of quantum physics, photons are the fundamental constituents of all forms of electromagnetic radiation, including light. They serve as the carriers of the electromagnetic force.
• Photons are elementary particles that have no mass and no electric charge. They are the quanta of the electromagnetic field, which is the fundamental field that describes electromagnetic interactions. Electromagnetic radiation, including light, is a manifestation of the electromagnetic field.
• Photons are the carriers of electromagnetic force because they are the only particles that can mediate electromagnetic interactions. When two charged particles interact electromagnetically, they exchange photons. The exchange of photons gives rise to the electromagnetic force.<
• Photons have no rest mass and always travel at the speed of light in a vacuum.
• Photons exhibit both wave-like and particle-like properties, a characteristic referred to as wave-particle duality. This duality is inherent to quantum particles, causing light to behave as a wave under certain conditions, as both waves and photons in others, and strictly as particles in yet others.