Dictionary tag: P

• Particle physics is an experimental sub-field of quantum mechanics often associated with the Large Hadron Collider at CERN.
• The Large Hadron Collider is a particle accelerator, which collides particles at high energies. These collisions create new particles, which are then studied by detectors. The detectors measure the properties of the new particles, such as their mass, energy, and charge.
• The Standard Model serves as the theoretical framework employed by particle physicists to investigate fundamental forces and particles.
• Among the most important discoveries of particle physics was the discovery of the photon and the dual nature of light (both a wave and particle).

• Our ability to perceive and distinguish between colours is crucial to how we experience and understand the world.
• Perceived colour is influenced by the range and mixture of wavelengths and intensities of light that enter the eye.
• Perceived colour can be influenced by the properties of light, objects, and the attributes of visual perception.
• Colour perception tends to prioritize information that is important to an observer, but it may not always be objectively accurate.
• The perceived colour of an object can be influenced by factors, such as the size, shape, and structure of objects, their position and orientation, and the direction of incident light.
• Colour perception can be described by chromatic colour names (such as pink, orange, brown, green, blue, and purple) and achromatic colour names (such as black, grey or white).
• Perceived colour is often a combination of chromatic and achromatic content.
• The state of adaptation of an observer’s visual system can affect colour perception, such as when fatigued cells in the retina produce after-images.
• An observer’s expectations, priorities, current activities, recollections, and previous experiences can all influence perceived colour.
• The International Commission on Illumination (CIE) defines perceived colour as having three main characteristics: hue, brightness (or lightness), and colourfulness (or saturation/chroma).
• Here at lightcolourvision.org we often characterise colour in terms of hue, saturation and brightness and so align our discussions with the HSB colour model.

Here is a short explanation of phosphorescence:

• Excitation: When a phosphorescent material is exposed to light, electrons within the material absorb the photons’ energy and move to a higher energy state (become excited).
• Trapped State: Unlike fluorescence, where electrons immediately return to their ground state and emit light, in phosphorescence, the excited electrons get “trapped” in a forbidden triplet state. This means they can’t directly transition back to their lower energy state.
• Gradual Release: Over time, the trapped electrons slowly find their way back to the ground state, releasing the stored energy as light. This process is much slower than fluorescence, which is why phosphorescence produces that lingering afterglow.

Key Points:

• Duration: Phosphorescence can last for seconds, minutes, or even hours, unlike fluorescence which ends almost immediately after the light source is removed.
• Excitation Source: Phosphorescent materials typically need light with shorter wavelengths for excitation, such as ultraviolet light.

Examples:

• Glow-in-the-dark toys: These are coated with phosphorescent materials.
• Safety signs: Phosphorescent signs stay visible in low light conditions.
• Watch dials: Some watch dials used to use phosphorescent paint for nighttime visibility (this has been replaced by safer alternatives in modern watches).

Photoluminescence in three Steps

1. Absorption: Light excites electrons in a material, pushing them to higher energy levels.
2. Relaxation: Excited electrons gradually lose energy.
3. Light Emission: Lost energy is emitted as light of a different wavelength (usually longer).

Key Points

• Emitted light often has lower energy (longer wavelength) than absorbed light due to energy loss.
• Different types of photoluminescence exist based on relaxation mechanisms:
  • Fluorescence: Quick relaxation, emitting light soon after absorption.
  • Phosphorescence: Slower relaxation, emitting light later, creating a “glow” effect.
  • Chemiluminescence: Light emission due to a chemical reaction. The reaction can be initiated by various factors, including but not limited to light absorption.

Applications

• Fluorescent lamps convert ultraviolet light to visible light.
• LEDs use electroluminescence (photoluminescence triggered by electricity).
• Biological markers help visualize structures or processes in cells.
• Quantum dots have bright, tunable photoluminescence for displays and solar cells.

• Measuring human visual responses to light is not straightforward because the eye is a complex and intricate organ.
• An internationally recognized system of measurements, known as the CIE system, was established in 1931 by the Commission Internationale de l’Eclairage (CIE).
• The Commission established the typical spectral responsiveness of the human eye to wavelengths across the visible spectrum and compiled the data into a photopic curve.
• The CIE’s photopic curve shows that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes, red and violet.
• A second set of measurements of the typical responsiveness of the human eye to wavelengths across the visible spectrum at low levels of light, (where determining colour differences is difficult), resulted in data compiled into the scotopic curve.
• Having defined the spectral response of the human eye, the CIE sought a standard light source to measure luminous intensity.
• Luminous intensity is a measure of how bright a light source appears to the human eye, and it is typically used to describe the brightness of light sources such as light bulbs, lamps, and LEDs.
• The first source, which led to the development of the terms footcandle and candlepower, was a specific type of candle commonly used in the 18th and 19th centuries before standardised artificial light sources were developed.
• In 1948, the standard was redefined for better repeatability. It was named the International Candle and defined as the amount of light emitted from a given quantity of melting platinum.

• 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.

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.

• The higher the photon’s frequency, the higher its energy. Equivalently, the shorter the photon’s wavelength, the higher its energy.
• Photon energy is determined solely by the photon’s frequency and wavelength.
• Other factors, such as the intensity of the radiation, do not affect the energy of individual photons. In other words, two photons of light with the same frequency have the same energy, regardless of their source. even if one was emitted from a wax candle and the other from the Sun.
• The electronvolt (eV) and the joule are the units commonly used to express the energy of photons.
• The energy of a photon can also be expressed:
  • In terms of its wavelength or frequency using Planck’s constant (h). E = hν = hc/λ, where E is the energy, ν is the frequency, λ is the wavelength, and c is the speed of light.
  • As a quantum of electromagnetic radiation.

• One of the most common interactions used to explain the link between electromagnetism and light (including visible light and other parts of the spectrum) is the photon-electron interaction.
• The specific outcome of a photon-electron interaction depends on the photon’s energy and the electron’s state. For example, if the photon has enough energy, it can free a bound electron from the atom. This is known as the photoelectric effect.
• If the photon does not have enough energy to eject the electron, it can be scattered by the electron. Compton scattering involves high-energy photons, like X-rays or gamma rays, where the photon transfers part of its energy to the electron. The photon loses energy (shifts to a longer wavelength) and changes direction, while the electron gains kinetic energy.
• On absorption of a photon by an electron, the electron gains energy and transitions to a higher energy level – a higher orbital around the nucleus of the atom.
• When an electron scatters a photon elastically, such as in Rayleigh scattering, the photon alters its trajectory but retains its energy, and the electron’s energy level remains unaffected. In Compton scattering, however, the electron gains energy.
• A photon can transfer all of its energy to an electron if it has more energy than the electron’s binding energy. In this case, the electron will be ejected from the atom with excess kinetic energy.
• The amount of energy a photon can transfer to an electron is capped by the photon’s own energy. A photon cannot impart more energy to an electron than it possesses.
• The likelihood of a photon being absorbed by an electron depends on both the photon’s energy and the electron’s energy level. Absorption occurs when the photon’s energy exactly matches the difference between two energy levels (resonant absorption). Generally, higher photon energy increases the absorption likelihood, while higher electron energy levels tend to decrease the absorption probability due to fewer available transitions.
• The likelihood of photon scattering is influenced by the photon’s energy and the electron’s energy state. In Compton scattering, the angle of photon-electron collision affects the energy loss of the photon. Polarization of the photon and electron spin can also influence specific types of scattering, but spin is less central in general scattering processes.

• Examples of a photons-electron interaction include:
  • Photoelectric Effect: When a photon with adequate energy impacts an electron, the electron can be expelled from the atom. Called the photoelectric effect, the photon’s energy must exceed the electron’s binding energy within the atom.
  • Compton Scattering: In a collision between a photon and an electron, the photon can scatter. The photon might relinquish energy during the collision, and the electron can acquire a portion of the photon’s energy.
  • Chromophore excitation: The photon-chromophore interaction accounts for the observable colour of objects.
    • The interaction between photons and chromophores is more intricate than photon-electron interaction. It encompasses energy redistribution among all electrons in a molecule and can result in various outcomes such as fluorescence, phosphorescence, and energy transfer.
    • The interaction between photons and chromophores is generally referred to as molecular excitation or chromophore excitation, distinct from photon-electron interaction but can still be considered a type of photon-electron interaction since it involves the interaction of photons with the electrons in chromophore molecules.