Photon properties

About the properties of photons
  • A photon is a type of elementary particle that is a quantum (plural = quanta) of the electromagnetic field. This means that it is the smallest quantity into which light can be divided.
  • A photon carries energy and can be described both in terms of a particle and a wave.
  • While the wave model of light works well for some phenomena, the particle model is necessary to explain others.
  • Light can exhibit both wave-like and particle-like behaviour depending on the experiment performed. This is known as wave-particle duality.
  • The wavelength of a photon determines its energy and frequency.
  • Photons with longer wavelengths have lower energy and frequency, while photons with shorter wavelengths have higher energy and frequency.
  • The wavelength of a photon can also affect its behaviour, such as its ability to penetrate materials or cause photochemical reactions.

Other properties of photons include:

  • Photons have zero rest mass but have energy and momentum proportional to their frequency.
  • Unlike other kinds of elementary particles, photons have no rest mass.
  • Photons are electrically neutral, meaning they have no electric charge.
  • Photons are stable particles that do not decay over time.
  • Photons can interact with other particles, such as electrons, through processes such as absorption and emission.
  • Photons can interact with other particles, such as electrons, through processes like absorption and emission.
  • Photons always travel at the speed of light in a vacuum, regardless of their frequency or energy.

Photons & electrons

About photons & electrons
  • Photons are fundamental particles and are considered to be the primary constituent of visible light and all other types of electromagnetic radiation.
  • Photons are considered to be pure energy, they have no mass or electric charge and are dimensionless.
  • Since photons exist at the subatomic level alongside other fundamental particles such as electrons, quarks, leptons and gauge bosons, they exist outside the range of our everyday perception.
  • However, when photons interact with electrons, the effects can be seen by the human eye.
  • For example, when light travels through air or water, it interacts with the electrons of the atoms and molecules it encounters, causing different wavelengths to scatter in various directions. This scattering of light is visible to the human eye when we see the blue colour of the sky or the red colour of a sunset.
  • If electrons had eyes, they would likely perceive photons as disturbances in the electric and magnetic fields that pervade the space around them.
  • In the event of a collision between a photon and an electron, the interaction might lead the electron to absorb the energy of the photon and transition to a higher energy state or to emit a new photon as it returns to a lower energy state.
  • These types of interactions are frequently described as “excitation” or “de-excitation” processes and can happen without any significant release of energy or the production of any visible effects.

Contemporary quantum mechanics brings another perspective to the way a photon interacts with an electron.

  • Photons are the carriers of electromagnetic force.
  • When a photon interacts with an electron, the interaction can take many different forms, depending on the energy and nature of the photon, and the specific properties of the electron and the surrounding environment.
  • In some cases, the interaction can cause the electron to take in the energy of the photon and move to a higher energy level, or to give off a new photon as it returns to a lower energy level.
  • The interaction between a photon and an electron must be described in terms of the probabilistic nature of quantum mechanics because the precise outcome of the interaction cannot be predicted with certainty. Only the probabilities of different outcomes can be calculated.
  • Other useful concepts that quantum mechanics brings to this interaction are:
  • Particle-Wave Duality: As both particles and waves, photons and electrons can be described using wavefunctions that give the probability of finding the particle at any point in space. The interaction isn’t deterministic, where you can exactly predict the outcome, but probabilistic, meaning you can calculate the likelihood of different outcomes.
  • Energy Levels and Quantum Jumps: The absorption of a photon’s energy by an electron is probabilistic. The electron has a certain probability to absorb the energy and “jump” to a higher level. If the electron is in a superposition of states (a fundamental concept in quantum mechanics), it may or may not absorb the photon depending on how the wavefunction collapses upon measurement.
  • Wavefunction: This provides the probabilities of the outcomes of measurements of a quantum system. It essentially describes everything that can be known about a quantum system. The act of measuring these properties causes the wavefunction to collapse to a particular state.
  • Wavefunction collapse: When a measurement is made on a quantum system, the wavefunction “collapses” into a state that is consistent with the outcome of the measurement. This state is one of the possible states that the system could have been in before the measurement, according to the superposition principle.
  • Superposition: According to quantum mechanics, particles can be in multiple positions and states simultaneously until a measurement is made, a property known as superposition.
    • Until a measurement is made, a particle doesn’t have a definite position but rather exists in a spread of possible positions.
    • Similar to positions, a quantum particle can exist in a superposition of other states. For example, an electron in an atom can exist in a superposition of spin states (spin up and spin down), or a superposition of energy states.
  • Uncertainty Principle: Because of the inherent uncertainty in position and momentum, the exact outcome of an interaction between a photon and an electron can’t be predicted with absolute certainty. We can only talk about probabilities.
  • Probability and Wavefunction Collapse: The interaction between the photon and the electron could result in various outcomes, each with its own probability, defined by the wavefunction. The exact outcome isn’t known until the wavefunction collapses upon measurement. This is inherently a probabilistic event.
  • Quantum Entanglement: If a photon and electron become entangled, the state of one particle will instantly affect the state of the other, no matter how far apart they are. This correlation is perfectly deterministic, but the outcome of measurements on each individual system is probabilistic, giving rise to the paradoxical nature of entanglement.

Photons, electric & magnetic fields

About photons, electric and magnetic fields
Photons & electric fields
  • Photons and electric fields are intimately connected in the framework of electromagnetic radiation.
  • When an electric field oscillates or changes, it generates electromagnetic waves.
  • This oscillation of the electric field gives rise to the emission of photons.
  • The energy of each photon is directly proportional to the frequency of the electric field oscillation.
  • Higher-frequency oscillations produce photons with higher energy, while lower-frequency oscillations produce photons with lower energy.
  • When photons interact with charged particles or materials with electric fields, they can be absorbed, transmitted, or scattered.
  • This interaction leads to phenomena such as the photoelectric effect, where photons can eject electrons from a material by transferring their energy to the electrons, and optical phenomena like reflection and refraction.
Photons & magnetic fields
  • When a photon interacts with a magnetic field, it can cause the magnetic field to oscillate, creating an electromagnetic wave.
  • This is because a magnetic field is one component of the electromagnetic field, which also includes an electric field.
  • When a photon interacts with the magnetic field, it transfers energy to it, causing the magnetic field to oscillate back and forth.
  • The oscillation of the magnetic field, in turn, creates an oscillating electric field, and the two fields together form an electromagnetic wave that propagates through space at the speed of light.
  • Fields refer to regions in space where a physical quantity is present that can exert a force or influence on other objects or particles.
  • Fields are used in physics to describe how certain properties or forces can vary with position and time.
  • In the case of electromagnetic fields, they represent the distribution of electric and magnetic properties in space.
  • An electric field is associated with electric charges and describes the force experienced by other charges (positive or negative) in its presence. Electric fields exist in the vicinity of charged objects and can exert forces on other charged particles.
  • A magnetic field is associated with moving electric charges, such as electric currents in wires, and it describes the force experienced by other moving charges (currents) in its vicinity. Magnetic fields exist around current-carrying conductors and can interact with moving charges, causing them to experience a magnetic force.
  • Electric, and magnetic forces, can be represented using vector fields.
    • In the case of an electric vector field, the vectors represent the electric force that a charged object would experience at different points in space due to the presence of other electric charges.
    • In the case of a magnetic vector field, the vectors represent the magnetic force that a moving charged object would experience at different points in space due to the presence of magnetic fields.
  • Fields are often represented in two dimensions using field lines.
    • The density of field lines indicates the strength of the field at a particular point – the more dense the lines, the stronger the field.
  • The conventions for how to show gravitational, electric, and magnetic field lines are all slightly different to model the unique aspects of each force. Some common models are shown below.

Speed of light waves & photons

About the speed of light waves & photons
Speed of light waves
  • The speed of a light wave is a measurement of how far it travels in a certain time.
  • The speed of light is usually measured in metres per second (m/s).
  • Light travels through a vacuum at a bit less than 300,000 kilometres per second.
  • The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
  • Light travels through other media at lower speeds.
  • A vacuum is a region of space that contains no matter.
  • Matter is anything that has mass and occupies space by having volume.
  • When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
  • In other contexts, empty space is not considered to be a medium because it does not contain matter.
  • When light is described in terms of photons rather than waves the following points are important:
Speed of photons
  • Light exhibits wave-particle duality, meaning it can be described as both a wave and a particle (photon).
  • Photons are massless particles that travel at the speed of light.
  • Photons carry energy and momentum in quantized discrete units.
  • “Quantized discrete units” refers to the way energy and momentum are carried by photons.
  • In quantum mechanics, certain physical properties, such as energy and momentum, are quantized, meaning they can only take specific discrete values rather than a continuous range of values.
  • For photons, this means that their energy and momentum come in distinct, non-continuous packets or “units.”