Dictionary tag: C

• In a wave diagram used to illustrate electromagnetic waves, a centreline may be used to show either:
  • The point where the electric and magnetic fields intersect, so illustrating the axis along which a wave propagates.
  • A point located halfway between the crest and trough of a wave, illustrating the axis along which a wave propagates.

• 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.

• 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.

• A chromaticity diagram has two axes, the x-axis and y-axis, that collectively display the comparative amounts of red, green, and blue light necessary to produce any colour.
• The most common chromaticity diagrams display the entire range of hues (at varying levels of saturation) within the colour space that a human observer can see 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 recognised 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 optimise the performance of equipment and materials used for colour reproduction.

• 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.

• The CIE XYZ colour space was the first comprehensive method able to systematise the relationship between colour stimuli and human colour perception.
• In an experimental situation, the CIE XYZ colour space is able to match any colour an observer sees with a known mixture of wavelengths of light.
• The foundation of the CIE XYZ colour space is the ability to identify the precise mixture of wavelengths of light needed to stimulate cone cells to produce any colour experience within the visible spectrum.
• Viewed diagrammatically the CIE XYZ colour space takes the form of a graph showing a volume of colour corresponding with every wavelength in the visible spectrum. The location of every colour is determined in relation to the x and y axes of the graph. The two axes are used to identify the coordinates for each colour within this two-dimensional vector space.
• The coordinates themselves are derived from tristimulus colour values.
• With the development of the CIE XYZ colour space, trichromatic colour models and their corresponding colour spaces provide methods for anticipating and managing colour reproduction in every applicable field and type of technology.
• In terms of colour management, the trichromatic colour theory underpins device-independent additive colour spaces such as the sRGB colour space and the Adobe RGB colour space and device-dependent additive colour models such as RGB, HSB and CMYK and their corresponding colour spaces.

The CIE XYZ colour space serves as a standard reference and underpins more recent colour spaces such as:

• CIELUV 1976 –  a modification of CIE 1931 XYZ used to display additive mixtures of light more conveniently.
• CIELAB 1976 –  a more perceptually linear colour space. Perceptually linear means that changes in colour values are directly related to changes in colour appearance.  CIELAB is commonly used for surface colours, but not for mixtures of light.

• Classical electromagnetism is based on the idea that electric charges and electromagnetic fields are continuous and smooth. It does not take into account the quantization of energy or the wave-particle duality of matter.
• Charged particles create electromagnetic fields, which in turn exert electromagnetic forces on other charged particles.
• The four Maxwell equations are:
  • Gauss’s law for electricity: The electric flux through a closed surface is proportional to the total electric charge enclosed by the surface.
  • Gauss’s law for magnetism: There are no magnetic monopoles, and the magnetic flux through a closed surface is always zero.
  • Faraday’s law of induction: A changing magnetic field produces an electric field.
  • Ampere’s circuital law with Maxwell’s correction: A changing electric field or an electric current produces a magnetic field.
• These equations can be used to describe a wide range of phenomena, from the propagation of electromagnetic waves to the operation of electrical and electronic devices. They are also used in many different fields, including engineering, medicine, and astronomy.

Core concepts of classical electromagnetism

• Charged Particles (Matter): These are particles that have an electric charge, either positive (protons) or negative (electrons). They are the sources of electric and magnetic fields and are affected by these fields.
• Electromagnetic Force: This force is a fundamental interaction between charged particles. It can be attractive or repulsive, depending on the sign of the charges.
• Electromagnetic Fields: These are regions where electric and magnetic forces are experienced due to the presence of charged particles. Electromagnetic fields carry energy and can exert forces on other charged particles.

Everyday examples of Maxwell’s electromagnetism

• When you turn on a light switch, the electric current in the filament of the light bulb produces a magnetic field. This in turn produces an electric field causing the filament to glow white hot.
• When you listen to the radio, the electromagnetic waves from the radio station interact with the antenna on your radio to produce an electric current. This electric current is then amplified and converted into sound, which you can hear through the speakers on your radio.
• When you use a microwave oven to heat food, the electromagnetic waves from the microwave oven interact with the water molecules in the food. This causes the water molecules to vibrate, which heats up the food.

• Classical physics describes many aspects of nature at an everyday scale but neglects to explain things at very small (sub-atomic) and very large (cosmological) scales. It is a very successful theory, and many of its predictions have been experimentally verified.
• Classical physics studies the motion of macroscopic objects, from projectiles to parts of machinery and astronomical objects such as spacecraft to the movement of planets and stars.
• For objects governed by classical physics, if the present state is known, it is possible to predict how it will move in the future (determinism), and how it has moved in the past (reversibility).
• Classical physics has its roots in:
  • Newtonian mechanics – Isaac Newton, 17^th century
  • Thermodynamics – Carnot, Joule and Kelvin, 19^th century
  • Maxwell’s electromagnetism, 19^th century.

• Newton’s laws of motion, the law of conservation of energy, and the law of conservation of momentum are all fundamental laws of Newtonian mechanics. Newtonian mechanics is a branch of physics that describes the motion of objects under the influence of forces.
• Thermodynamics is a branch of physics that deals with the relationship between heat and work. Thermodynamics is based on the two laws of thermodynamics, which were developed in the 19th century by Carnot, Joule, and Kelvin.
• Maxwell’s electromagnetism is a branch of physics that describes the interaction of electric and magnetic fields. It is based on the four Maxwell equations, which were developed by James Clerk Maxwell in the 19th century.
• These three branches of physics are all related to each other. Newtonian mechanics can be used to describe the motion of objects in thermodynamic systems, and thermodynamics can be used to describe the energy changes that occur in electromagnetic systems.

Here are some examples of how these three branches of physics are related:

• The heat engine is a thermodynamic device that converts heat into work. The efficiency of a heat engine is limited by the laws of thermodynamics and the laws of Newtonian mechanics.
• The electric generator is an electromagnetic device that converts mechanical energy into electrical energy. The operation of an electric generator is based on the laws of electromagnetism and the laws of Newtonian mechanics.
• The light bulb is an electromagnetic device that converts electrical energy into light energy. The operation of a light bulb is based on the laws of electromagnetism and the laws of thermodynamics.
• Classical physics has some limitations. For example, classical physics cannot explain the behaviour of light at the atomic and subatomic levels. This is because light behaves both like a wave and a particle, which is something that classical physics cannot describe. These limitations are explored in the field of quantum mechanics (quantum physics).

• The primary colours in the CMY colour model are cyan, magenta and yellow.
• The CMY colour model is a subtractive colour model used with transparent or translucent inks or filters.
• The CMY colour model along with its system of notation enables an exact and reproducible approach to colour printing and other similar applications.
• The CMY colour model is deeply embedded in all contemporary digital printer technologies and underpins industrial standards for the printing industry.

Subtractive colour mixing

• The CMY colour model can be explained by imagining that an observer is presented with a well-illuminated surface such as a highly reflective sheet of white paper.
• In the diagram below a torch is used to illuminate the paper, producing a round pool of light.
• The surface appears white because it is illuminated by white light, so by light containing all the wavelengths of the visible spectrum.
• Cyan, magenta and yellow filters or inks are then placed between the light source and the paper or between the paper and the observer.
• The diagram below shows the effect of placing the filters or patches of ink onto the paper so they partially overlap.

• Where pairs of the primary coloured C, M and Y filters/inks overlap they produce secondary colours and where all three overlap, all wavelengths of light are blocked producing a dark area in the centre.
  • A red filter will transmit red light but absorbs all other colours including green and blue.
  • A green filter will transmit green light but absorbs all other colours including blue and red.
  • A blue filter will transmit blue light but absorbs all other colours including red and green.

• Where two overlapping filters are placed between the light source and the paper or between the paper and the observer the results are as follows:
  • A red filter absorbs both green and blue and a green filter absorbs blue and red, as a result, red, green and blue are all absorbed where they overlap and that area appears black.
  • A green filter absorbs both blue and red and a green filter absorbs blue and red, as a result, red, green and blue are all absorbed where they overlap and that area appears black.
  • A blue filter absorbs both red and green and a red filter absorbs green and blue, as a result, red,  green and blue are all absorbed where they overlap and that area appears black.

• Where all three filters are placed between the light source and the paper or between the paper and the observer the result is that red, green and blue are all absorbed where they overlap and that area appears black.

• Cyan, magenta and yellow filters that correspond with the secondary colours in the RGB colour model but are the primary colours in the CMY colour model behave as follows.
  • A cyan filter absorbs red but transmits green and blue light. Green and blue together appear cyan to the human eye.
  • A magenta filter absorbs green but transmits red and blue light. Red and blue together appear magenta to the human eye.
  • A yellow filter absorbs blue but transmits red and green light. Red and green together appear yellow to the human eye.

• Lastly, where two overlapping CYK filters are placed between the light source and the paper or between the paper and the observer so that they overlap, the results are as follows:
  • A cyan filter transmits green and blue light whilst a magenta filter transmits red and blue. Green and red cancel out producing blue.
  • A magenta filter transmits red and blue light whilst a yellow filter transmits red and green. Blue and green cancel out producing red.
  • A yellow filter transmits red and green light whilst a cyan filter transmits green and blue light.  Red and blue cancel out producing green.

CMYK colour model

• The CMY colour model is helpful in developing an understanding of how combinations of cyan, magenta and yellow primary colours can be used to produce a wide range (gamut) of colours when light is reflected off a surface and wavelengths of light are filtered out by the inks before and reaching the eyes of an observer.
• The CMYK colour model (sometimes called four-colour or process printing) uses the same three primary colours as CMY but uses a fourth component, black ink (K), to increase the density of darker colours and blacks.
• CMYK printing typically relies on:
  • Using white paper with good reflective properties to produce the brightest possible highlights by reflecting the maximum amount of light back towards the observer.
  • Creating highlights by using the minimum amount of coloured ink and printing without black.
  • Producing fully-saturated mid-tones by relying on the brilliance and transparency of printing inks and dyes.
  • Adding black ink when the maximum amounts of cyan, magenta and yellow are insufficient to produce rich black tones in areas of shadow and where black text is required.

Half-tone printing with CMYK

• CMYK is based on the CMY colour model and is the standard model used for colour printing.
  CMYK refers to the four ink plates used in colour printing: cyan, magenta, yellow, and a ‘key’ colour (black).
• Half-tone printing (half-toning) using CMYK involves printing tiny dots of CMY and K in a pattern small enough that humans see solid areas of colour.
• Half-toning allows for a continuous variation in the colour perceived by a viewer through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.
• Half-tone printing can be used to reproduce black-and-white images by using only black ink and varying sizes or spacing of the dots. This simulates shades of grey, allowing for the representation of continuous tones and gradients in black-and-white images.
• Half-tone black-and-white printing is widely used in newspapers, magazines, and other printed materials.
• The term halftone is sometimes also used to refer specifically to the image that is produced by half-tone printing.

• CMYK refers to the four ink plates used in some colour printing: cyan, magenta, yellow, and ‘key’ (black).
• The CMYK model works by overlaying colours that partially or entirely mask the background colour (usually white). The ink reduces the light that would otherwise be reflected.
• CMY and CMYK are called subtractive colour models because the inks “subtract” the colours red, green and blue from white light.
• Remember that when an observer looks at an image printed using CMYK inks, they are looking at the light that has first fallen on and then been reflected off the surface they have been applied to.
• When incident light contains all the wavelengths of the visible spectrum (white light) is reflected from areas of the image that have no ink on them an observer sees white highlights in the images.
• When white light is reflected from areas of the image that have cyan ink on them the observer sees red.
• When white light is reflected from areas of the image that have magenta ink on them the observer sees green.
• When white light is reflected from areas of the image that have yellow ink on them the observer sees blue.

• We see colour as a result of:
  • The range and intensities of wavelengths of visible light emitted by a light source.
  • The path that light takes and the different media and materials it encounters on the journey from its source to the retina of a human eye.
  • Optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
  • An observer’s predispositions, including their personal and social experience and cultural context.