###### 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.

###### 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.

###### Pigment epithelium

Pigment epithelium is a layer of cells at the boundary between the retina and the eyeball that nourish neurons within the retina. It is firmly attached to the underlying choroid is the connective tissue that forms the eyeball on one side but less firmly connected to retinal visual cells on the other.