Dictionary tag: C

• In a wave diagram used to illustrate electromagnetic waves, a centreline may be used to show either:
  • Point of intersection: This is the ideal centerline and represents the point where the electric and magnetic fields cross zero simultaneously. This point stays constant as the wave propagates.
  • Halfway between crest and trough: This is a common but simpler representation used for ease of visualization. It doesn’t always coincide with the point of field intersection in certain wave types or when considering polarization.

• Electric charge carriers, protons (+) and electrons (-) are the primary charge carriers in matter.
• There are two types of electric charge:
  • Positive charge: Carried by protons, found in the nucleus of atoms.
  • Negative charge: Carried by electrons, which exist in orbitals around the nucleus.
• Neutons, the other particles within the nucleus of an atom, have no charge.

Properties of electric charge

• Like charges repel, opposite charges attract: Particles and objects with the same type of charge repel each other, while objects with opposite charges attract each other.
• Electrostatic force: The force between charged objects is called the electrostatic force, and it’s governed by Coulomb’s law.
• Conservation of charge: The total electric charge in an isolated system is always conserved. It cannot be created or destroyed, only transferred between particles or regions.
• This means, for example, that even though electrons can move between energy levels within an atom, the total number of protons (positive charges) and electrons (negative charges) remains constant, upholding the principle of conservation of charge.

Examples of charge

• Static electricity: Rubbing a balloon on your hair transfers electrons, creating a static charge that can make hair stand on end or attract small objects.
• Electric current: The flow of electrons through a conductor, like a wire, creates an electric current, which can be used to power our devices.
• Lightning: Lightening is a dramatic example of charge-discharge in nature, caused by the buildup of static electricity in clouds.

About electric charge and energy in an atom

• An atom has a set number of particles that determines what kind of element they are.
• Each element has a specific number of protons (positive charge) in its nucleus. It is this number that defines an element and cannot change in everyday situations.
• Protons in the nucleus of an atom are very stable and don’t typically move around or get created or destroyed. There are a fixed number and they maintain their positive charge within the atom.
• Electrons (negative charge) surround the nucleus, with the number typically matching the number of protons to create a neutral atom.
• Regardless of their energy level, an atom retains the same number of electrons it started with.
• If an atom loses or gains an electron, it is no longer considered the same element and becomes an ion.
• The movement of electrons within an atom doesn’t change their total charge because the number of protons and electrons remains constant. However, the movement of electrons does affect the amount of energy within the atom.
• Electrons in an atom change energy levels as they gain and lose energy.
• So when an electron absorbs energy (such as visible light), it jumps to a higher energy level further away from the nucleus. However, the electron itself remains negatively charged, it just occupies a different position within the atom.

• Charged particles are the fundamental building blocks of matter. They include electrons, protons, and neutrons, which make up atoms, as well as ions, which are atoms that have lost or gained electrons. Charged particles also include more exotic particles, such as muons and pions, which are found in cosmic rays and in the decay of other particles.
• The electric charge of a particle is measured in coulombs (C). An electron has a charge of -1.6×10^-19 C, while a proton has a charge of +1.6×10^-19 C. Neutrons are neutral and have no charge.
• Charged particles interact with each other through the electromagnetic force, which is one of the four fundamental forces of nature. The electromagnetic force is responsible for the attraction between oppositely charged particles and the repulsion between like-charged particles. It is also responsible for the behaviour of electric and magnetic fields.
• Charged particles are also affected by magnetic fields. A magnetic field exerts a force on a moving charged particle, which can cause the particle to change its direction or speed. This is how electric motors work.
• A moving charged particle produces both an electric and a magnetic field. This is because a charged particle will always produce an electric field, but if the particle is also moving, it will produce a magnetic field in addition to its electric field.
• The magnetic field is always perpendicular to both the direction in which the charge is moving as well as to the direction of the electric field.

• The material world is bound together by chemical bonds, which determine the structure, size and characteristics of chemical compounds.
• A chemical compound consists of two or more atoms from different elements that are chemically bonded together.
• Chemical bonds occur because the electromagnetic force operates between charged particles.
  • Opposite charges attract one another and like charges repel.
  • The higher the charge, the stronger the force.
  • There are different types of chemical bonds. Each affects the physical and chemical properties of a compound, including reactivity, melting point, boiling point, and electrical conductivity.

The most common types of chemical bonds are:

Covalent Bonds

• Covalent bonds occur when electrons are shared between two atoms. The shared electrons are attracted to the protons of both nuclei, which keeps the atoms bonded together.

Ionic Bonds

• Ionic bonds occur when one atom completely transfers one or more electrons to another atom. This creates ions, with the atom that loses electrons becoming a positively charged ion and the atom that gains electrons becoming a negatively charged ion. The attraction between these opposite charges keeps the ions bonded together.

Metallic Bonds

• Metallic bonds are found in metals; they consist of the electrostatic attractive force between the conduction electrons, in what is known as an electron cloud, and the positively charged metal ions. These bonds allow for characteristics such as high melting points, malleability, and conductivity.

Hydrogen Bonds

• Hydrogen bonds are a type of dipole-dipole interaction that happens when a hydrogen atom bonded to a strongly electronegative atom (like nitrogen, oxygen, or fluorine) is also attracted to another electronegative atom in the same or another molecule.

Key features of chemiluminescence

• Energy source: Chemical reactions release energy in various forms, including light. This energy excites electrons in chemiluminescence, leading them to higher energy levels.
• Electron transitions: Excited electrons return to their ground state, releasing excess energy as light. This emission determines the light’s colour.
• Examples: Glow sticks, luminous fungi, and deep-sea organisms.
• Variations: Emitted light characteristics (intensity, colour, duration) depend on the specific chemical reaction.

Mechanisms of chemiluminescence

• Electron transfer: One reactant loses an electron (oxidation), the other gains an electron (reduction). This transfer can excite an electron in the accepting molecule, leading to light emission.
• Excited intermediates: Some reactions create intermediate molecules in excited states. These release excess energy as light when returning to their ground state.
• Free radicals: Highly reactive free radicals can undergo rearrangements or reactions with other molecules, releasing energy as light.

Applications

• Chemical analysis: Detecting specific substances based on unique light emission patterns of certain reactions.
• Biological research: Studying biological processes involving chemiluminescent molecules in organisms.
• Safety devices: Light sticks for emergency lighting or marking locations use chemiluminescent reactions.

• Chromatic dispersion denotes the process of separation according to colour.
• Chromatic dispersion is the result of the connection between wavelength and refractive index..
• When light moves from one medium (like air) to another (like water or glass), each wavelength is influenced to a varying extent based on the refractive index of the involved media. The outcome is that every wavelength changes its direction and speed. If the light source emits white light, the individual wavelengths spread out, with red at one end and violet at the other.
• A familiar example of chromatic dispersion is when white light strikes raindrops and a rainbow becomes visible to an observer.
• Keep in mind that wavelength is a characteristic of electromagnetic radiation, whilst colour is an aspect of visual perception.

• Chromaticity refers to the quality of a colour that sets it apart from white, grey, or black.
• The chromaticity of different colours is often described by chromaticity coordinates that define where a colour appears within a colour space.
• The simplest way to understand chromaticity is through a chromaticity diagram that creates a two-dimensional visual display of all the colours produced by a specific colour space.
• A chromaticity diagram displays hue and saturation without mentioning their brightness.
• The most common chromaticity diagrams showcase the full range of colours visible to a human observer under ideal conditions. The position of each colour is plotted using the range of colour values (chromaticity coordinates) described by the CIE (1931) XYZ colour space.
• Some chromaticity diagrams illustrating the CIE (1931) XYZ colour space include overlays of the smaller gamuts of colour spaces associated with different mediums, lighting conditions, and devices.
• Examples of colour spaces with smaller gamuts than the CIE (1931) XYZ colour space include:
  • Adobe RGB (1998)
  • Prophoto RGB
  • sRGB
  • 2200 matt paper

• This means it shows the range of colours achievable by combining red, green, and blue light in varying proportions, not all possible colours imaginable. Some chromaticity diagrams may include colours that are technically visible under specific conditions (e.g., high intensity) but are not typically seen by humans under normal viewing conditions.
• The two axes in a chromaticity diagram, typically labelled x and y, represent the proportions of red, green, and blue light needed to produce a specific colour within the model’s gamut.
• The most common diagrams, like the CIE 1931 xy diagram, display the range of hues (at varying saturation levels) that a human observer can perceive under ideal conditions.
• The scale on each axis of chromaticity diagrams used for technical purposes aligns with the range of colour values (chromaticity coordinates) described by the CIE (1931) XYZ colour space. This enables them to accurately depict colour spaces in a manner consistent with a comprehensive and internationally recognized chromaticity coordinate system.
• Some chromaticity diagrams show the smaller range of other colour spaces so that the range of colours that can be reproduced by equipment such as cameras, digital screens and printers can be compared.
• Chromaticity diagrams are used to:
  • Ensure predictable, consistent and accurate colour reproduction across different devices and platforms.
  • Compare the chromaticity of colours, and so determine the difference between the appearance of particular colours or ranges of colour in terms of hue and saturation.
  • Assess and optimize the performance of equipment and materials used for colour reproduction.

Chromaticity diagram description

• The colours in a chromaticity diagram appear on a horseshoe-shaped block positioned between two axes and taken as a whole, represents the entire range of colours a person with average eyesight perceives.
• Each point within the horseshoe signifies the position of a unique spectral colour in terms of its hue and saturation.
• In theory, a high-quality HD display can show a chromaticity diagram containing over 16 million colours.
• The scales on the x and y axes can be used to plot the component RGB values associated with any colour.
  • As the values increase along the x-axis, the amount of blue in a colour decreases as red increases.
  • As the values increase along the y-axis, the amount of blue in a colour decreases as green increases.
  • As values increase along both the x and y axis the diagram locates every possible colour.
• Fully saturated colours appear along the boundary of the horseshoe and every pixel corresponds with the wavelength of a single spectral colour measured in nanometres.
• The fully saturated spectral colours along the boundary become less saturated towards the centre of the diagram.
• The circle in the centre, the white point, indicates the point where amounts of red, green and blue add together to produce a neutral white.
• The white point represents the appearance of natural light at midday.
• Two-dimensional chromaticity diagrams don’t show the z-axis which corresponds with colour brightness.

• Things appear to have colour because they absorb certain wavelengths of light while reflect others.
• When wavelengths of light within the visible spectrum enter the human eye, the observer perceives this as colour.

• The chromophore is the part of a molecule where there is an energy difference between two different molecular orbitals.
• A molecular orbital refers to the position and wave-like behaviour of an electron as it moves around an atom’s nucleus.
• If the energy difference of a chromophore falls within the range of the visible spectrum (2 to 2.75 electron volts) then it will produce colour.
• The colour produced by a surface or object corresponds with wavelengths of light that are not absorbed during their interaction with the chromophore.