Knowledge-base tag: C

A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.

• A chemical compound consists of two or more atoms from different elements chemically bonded together.
• There are two types of chemical bond: covalent bonds and ionic bonds:
  • A covalent bond forms when two atoms share a pair of electrons.
  • Atoms can lose or gain electrons in chemical reactions. When they do this they form charged particles called ions.
• Chemical bonds occur because opposite charges attract via the electromagnetic force.
• Negatively charged electrons orbiting the nucleus of an atom and the positively charged protons in the nucleus attract each other.
• An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond.
• Due to the matter-wave nature of electrons and their smaller mass, they must occupy a much larger amount of volume compared with the nuclei, and this volume occupied by the electrons keeps the atomic nuclei in a bond relatively far apart, as compared with the size of the nuclei themselves.
• The physical world is held together by chemical bonds, which dictate the structure and the bulk properties of matter.

https://en.wikipedia.org/wiki/Chemical_bond

Things appear coloured because colour corresponds with a property of light that is visible to the human eye. The visual experience of colour is associated with words such as red, blue, yellow, etc.

The colour an observer sees depends on:

• The range of wavelengths and intensity of each component of visible light emitted by a light source.
• The path that the light takes.
• The different media, materials or objects it encounters.
• Optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.

Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space. Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.

The experience of colour is a feature of human vision that depends first of all on the construction of our eyes and the wavelength, frequency and amplitude of visible light that strikes the retina at the back of each eye.

Because colour is a visual experience that is specific to each and every one of us at any given moment, we can try and share our experiences of colour using language but colour cannot be defined without examples.

The name given to light that contains all wavelength of the visible spectrum is white light.

When white light strikes a neutral coloured object, and all wavelengths are reflected, then it appears white to an observer.

The term white light doesn’t mean light is white as it travels through the air.

As light travels through the air it is invisible to our eyes.

The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.

The colour an observer sees depends on the wavelengths of visible light emitted by a light source and on which of those wavelengths are reflected off an object.

Although a human observer can distinguish between many thousands of wavelengths of light in the visible spectrum our brains often produce the impression of bands of colour.

As light travels from one medium to another, such as from air to glass, the wavelength changes but the frequency remains the same so the colour seen by an observer remains the same.

https://en.wikipedia.org/wiki/Color

Colour constancy refers to the ability of the human eye and brain to automatically compensate when objects change colour because of changes in illumination.

• Colour vision relies on colour constancy to enable an observer to perceive the colour of an object as almost unchanged as levels of illumination change and the spectral distributions of light changes.
• A human observer will often not notice when the colour of object changes as the source of illumination changes e.g. sunlight to artificial light.
• Colour vision allows us to distinguish different objects by their colour. In order to do so, colour constancy can keep the perceived colour of an object relatively unchanged when the illumination changes among various broad (whitish) spectral distributions of light.
• Colour constancy is achieved by chromatic adaptation. The International Commission on Illumination defines white (adapted) as “a colour stimulus that an observer who is [chromatically] adapted to the viewing environment would judge to be perfectly achromatic and to have a luminance factor of unity. The colour stimulus that is considered to be the adapted white may be different at different locations within a scene.
• The effect of changes in colour balance is very noticeable when comparing photographs of the same subject taken in different lighting conditions. Cameras use white balance to compensate for changes in illumination.

https://en.wikipedia.org/wiki/Color_constancy

The most common forms of colour notation used with computer software and by digital equipment are the triplets of the  RGB and HSB colour models and the quadruplets used by CMYK model.

• RGB notation can be represented in decimal or hexadecimal formats.
  • Decimal RGB notation for orange is: R=255, G=128, B=0.
  • Hexadecimal RGB notation  for orange is: #FF8000.
• HSB notation is represented in a decimal format.
  • The HSB notation for orange is: H=30.12, S=100, B=100.
• CMYK notation is represented in a decimal format.
  • The CMYK notation for orange is: C=0, M=61.48, Y=100, K=0.

A material gets its colour as electrons absorb some wavelengths of light and reflect others. The colour an observer sees corresponds with the reflected wavelengths.

• Three key factors affect the colour of an object.
  • • The light source and what happens to the light on its journey towards an object.
    • What happens when light strikes a material or medium.
    • Factors related to the observer which affect what they see.

Colour vision is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.

• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a range of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
• Colours can be measured and quantified in various ways. But an observer’s perception of colour is a subjective process whereby their brain responds to stimuli that are produced when incoming light reacts with light-sensitive cells at the back of their eye. As a result, different people may see the same illuminated object or light source in different ways.

https://en.wikipedia.org/wiki/Color_vision

A colour wheel is a diagram based on a circle divided into segments. The minimum number of segments is three with a primary colour in each. Segments added between the primaries can then be used to explore the result of mixing adjacent pairs of primary colours together. Additional segments can then be added between all the existing segments to explore the result of mixing further pairs of adjacent colours.

• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of rainbow colours.
• Colour wheels are often used in technologies which reproduce colour in ways that match the light sensitivity of the three different types of cone cells and the rod cells in the human eye.
• Colour wheels exploring additive colour models and subtractive colour models use different sets of primary colours.
• An RGB colour wheel, used to explore additive mixing of light, starts with red, green and blue primary colours.
• The colours produced in between the primary colours in a colour wheel are called secondary colours.
• The colours produced in between the secondary colours in a colour wheel are called tertiary colours.
• A CMY colour wheel, used to explore subtractive mixing of pigments and inks (used in digital printing) starts with cyan, magenta and yellow primary colours.
• An RYB colour wheel used to explore the subtractive mixing of art pigments and paints starts with red, yellow and blue primaries.
• The colour wheels described above all depend on trichromatic colour vision which involves three receptor types (cone cells) processing colour stimuli.

https://en.wikipedia.org/wiki/Color_wheel

In the context of a discussion of light (as opposed to pigments) complementary colours are two colours that, when mixed together, produce white light.

• When working with the RGB colour model, combining the wavelengths corresponding with red, green and blue primary colours produces white for a human observer.
• The complementary colour of each primary colour when working with the RGB colour model is the secondary colour produced by combining the other two primaries.
• The complementary primary–secondary combinations of light are red–cyan, green–magenta, and blue–yellow.
• Combinations of complementary primary–secondary colours at full intensity make white light because together they contain wavelengths corresponding with all three primaries.
• A complementary colour plus a primary colour combine to produce white because each complementary colour is the product of combing two primaries.
• So a complementary colour produced by combining green and blue primaries makes white when combined with red.

https://en.wikipedia.org/wiki/Complementary_colors

A compound is a substance made from the combination of two or more elements and held together by chemical bonds that are difficult to break. The bonds form as a result of sharing or exchanging electrons among atoms. The smallest unbreakable unit of a compound is a molecule.

• A compound is formed when different elements react with each other, forming bonds between atoms that produce molecules.  When a compound is exposed to a new element further reactions can take place which produce new compounds.
• A compound differs from a mixture because the atoms in a mixture are not bonded together. In this case, different elements mix together but no chemical reaction takes place, so each element remains separate and distinct.

Cone cells, or cones, are one of three types of photoreceptor cells (neurons) in the retina of the human eye. They are responsible for colour vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.

• The three types of photoreceptor cells (neurons) in the retina of the human eye are cone, rod and ganglion cells.
• Cone cells are cone-shaped whilst rod cells are rod-shaped.
• Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
• There are believed to be around six million cone cells and 90 million rod cells in the human retina.

https://en.wikipedia.org/wiki/Cone_cell

A continuous spectrum, emitted by a light source such as the sun, consists of a continuum of wavelengths of electromagnetic radiation across the visible spectrum.

• A continuous spectrum is produced by light emitted by excited atoms. It results from free electrons becoming bound to ions which then emit photons.
• In the cse of sunlight the emitted photons are smoothly spread over a wide range of wavelengths.

The crest is the point on a wave with the maximum value of upward displacement within a wave-cycle. A trough is the opposite of a crest, so the minimum or lowest point in a wave-cycle.

• On a wave at sea, the crest is a point where the displacement of water is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the medium is at a minimum.
• In the case of an electromagnetic wave which has an electric and a magnetic axis then a crest on either axis refers to maximum displacement in the positive direction whilst a trough refers to minimum displacement.

https://en.wikipedia.org/wiki/Crest_and_trough

The crest is the point on a wave with the maximum value of upward displacement within a wave-cycle. A trough is the opposite of a crest, so the minimum or lowest point in a wave-cycle.

• On a surface wave, the crest is a point where the displacement of the medium (water for example) is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the medium is at a minimum.
• In the case of an electromagnetic wave which has an electric and a magnetic axis then a crest on either axis refers to maximum displacement in the positive direction whilst a trough refers to minimum displacement.

https://en.wikipedia.org/wiki/Crest_and_trough