Dictionary tag: S

Substance

A substance is a type of matter with uniform properties throughout. This means that a sample of a substance will have the same characteristics regardless of its size.

  • One kind of substance is a chemical substance. A chemical substance is a specific type of matter with molecules that share the same structure and composition.
  • These molecules are held together by chemical bonds. Substances cannot be separated into their component parts (elements or compounds) without breaking these chemical bonds.
  • Substances can be generally classified into two main categories:
    • Elements: The simplest form of a substance, elements are made up of only one type of atom.
    • Compounds: These substances are formed when two or more elements chemically bond together.
  • The properties of a substance, such as melting point, boiling point, and density, are unique and can be used to identify the substance and distinguish it from other substances. Chemical reactions involve the formation and breaking of these chemical bonds, transforming one or more substances into different substances with new properties.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Subtractive colour model

A subtractive colour model combines different hues of a colourant such as a pigment, paint, ink, dye or powder to produce other colours.

  • CMYK is a subtractive colour model.
  • CMYK pigments are the standard for colour printing because they have a larger gamut than RGB pigments.
  • CMYK printing typically uses white paper with good reflective properties and then adds cyan, magenta, yellow and black ink or toner to produce colour.
  • Highlights are produced by reducing the amount of coloured ink and printing without black to allow the maximum amount of light to reflect off the paper through the ink.
  • Mid tones rely on the brilliance and transparency of the pigments and the reflectivity of the paper to produce fully saturated colours.
  • Shadows are produced by adding black to both saturated and desaturated hues.

 

Subtractive colour model

A subtractive colour model explains how different coloured pigments (such as paints, inks, dyes or powders) mix to produce other colours. This concept applies primarily to opaque objects, which don’t allow light to pass through them. Subtractive colour mixing can also be observed with translucent materials that partially transmit light.

  • Widely used subtractive colour models include:
    • CMY colour model: This model is a theoretical foundation for understanding how cyan, magenta, and yellow inks combine to create a wide range of colours.
    • CMYK colour model: This is a practical application of CMY, used in printing. It adds black ink (K) to the CMY combination for better contrast and richer blacks, especially when printing on highly reflective surfaces.
    • RYB colour model: This model is a historical approach that uses red, yellow, and blue pigments to teach colour-mixing concepts.
Subtractive colour with opaque pigments
  • All subtractive colour models rely on the principle that the colour of an opaque object or surface is determined by the wavelengths of light it absorbs and the wavelengths it reflects.
  • When light hits an object, some wavelengths are absorbed by the material, and the remaining reflected wavelengths are perceived by our eyes as colour.
  • Mixing opaque pigments creates a subtractive effect because each pigment absorbs a specific range of wavelengths.
  • As more pigments are combined, they absorb even more wavelengths of light.
  • This reduces the amount of light reflected to our eyes, resulting in a new colour perception.
  • Additionally, mixing contrasting colours like red and green (which absorb opposite ends of the light spectrum) leads to a darker result because they absorb a wider range of wavelengths combined.
Subtractive colour with translucent pigments
  • For translucent inks and dyes applied to a material like paper, the observer perceives a mixture of two things:
    • Reflected wavelengths of light: Similar to opaque pigments, translucent materials reflect certain wavelengths of light that determine the perceived colour.
    • Transmitted light reflected by the paper: Some light passes through the translucent pigment layer and reflects from the surface beneath, such as the paper. This adds another layer of reflected light to the overall colour perception.
  • The colour an observer sees ultimately depends on both the properties of the translucent ink or dye and the properties of the underlying material (like paper). Here are some factors that can influence the final colour:
    • Surface finish of the paper: A smooth paper surface might cause more light reflection compared to a textured surface.
    • The angle of incoming light: The angle at which light hits the translucent material can affect how much light passes through and how much reflects.
    • Viewing angle of the observer: Depending on the angle from which you look at the coloured material, the interplay of reflected and transmitted light can cause the colour to appear slightly different.
About the CMY colour model and colour perception
  • A good starting point for understanding the CMY colour model is trichromatic colour theory.
    • Trichromatic colour theory explains the underlying physiological basis for the subjective experience of colour.
    • Trichromatic colour theory and its precursors have established that there are three types of cone cells (recognised by the initials L, M and S) in the human eye that carry out the initial stage of colour processing that ultimately produces the world of colours we see around us:
      • L = Long (500–700 nm)
      • M = Medium (440 – 670 nm)
      • S = Short (380 – 540 nm)
  • Trichromatic colour theory also states that three monochromatic light sources, one red, one green, and one blue, when mixed together in different proportions, can stimulate the L, M, and S cones to produce the perception of any colour within the visible spectrum.
  • All colour models, such as the RGB and CMY models, have their foundations rooted in the trichromatic principles of human vision
No posts found.
About subtractive colour printing in practice
  • CMY printing involves three translucent inks corresponding with the primary colours – cyan, magenta and yellow.
  • The CMY colour model is subtractive in the sense that each primary colour can subtract from the light that reaches an observer’s eyes.
  • In CMY colour printing,  colour is applied to the surface of a medium either as dots or as solid areas of colour.
  • The CMY colour model doesn’t define the exact hue of the three primary colours, so when experimenting with real inks, the results depend on how they are made.
CMY on a white sheet of paper
  • Cyan ink is painted onto the paper to create a circular shape.
  • The paper seen through the cyan ink appears cyan to an observer because:
    • The ink has absorbed or transmitted all wavelengths of light except those around 500 nanometres (cyan).
    • The wavelengths of light around 500 nanometres reflected off the ink, making it look cyan.
    • Some transmitted wavelengths passed straight through the ink, reflected off the paper below, passed back through the ink, and added to the intensity of the colour seen by the observer.
  • Matching patches of magenta and yellow are now painted onto the paper so that areas of each of the three colours overlap.
  • As already established,  the paper seen through the yellow ink alone appears yellow because it has absorbed all wavelengths of light other than those around 500 nanometres (cyan).
  • Whilst the paper seen through the magenta ink alone appears magenta because it has absorbed all wavelengths of light other than those around 700 nanometres (red).
  • And the paper seen through the yellow ink alone appears yellow because it has absorbed all wavelengths of light other than those around 580 nanometres (yellow).
  • Where cyan and magenta ink overlap, the paper appears blue. This is because the cyan ink absorbs red light and allows blue light to pass through, while the magenta ink absorbs green light.
  • Where magenta and yellow ink overlap, the paper appears red. This happens because the magenta ink absorbs green light and lets red and blue light pass through, while the yellow ink absorbs blue light, leaving only the red light.
  • Where yellow and cyan ink overlap, the paper appears green. This occurs because the yellow ink absorbs blue light and allows green and red light to pass through, while the cyan ink absorbs red light, leaving only the green light.
  • Where all three inks overlap the paper appears dark brown.
  • Remember that in practice, a fourth ink, black (K), is often added to the CMY model to create the CMYK model, which provides better depth and detail in dark areas and helps save ink.
  • CMYK is commonly used in printing processes like inkjet and laser printing, as well as offset printing for large-scale projects.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Subtractive colour on screen

About subtractive colour on screen
  • Computers, TVs and phones use the additive RGB colour model to represent colour. It’s called additive because it works by adding different coloured lights together to create new colours.
  • Although all the displays on these devices use the RGB colour model, they can still be used to explore the effects of subtractive colour models such as CMY or RYB.
  • Subtractive colour models used in printing work by subtracting colours from white light to produce different hues.
  • In the CMY model, the primary colours are cyan, magenta, and yellow, while in the RYB model, the primary colours are red, yellow, and blue.
  • By using a computer, TV, or phone to explore subtractive colour models, it’s possible to visualize how colours are created by subtracting different wavelengths of light.
  • The ability to explore subtractive colour models using computers, TVs, and phones can be useful for designers, artists, and anyone working in the printing industry, as it allows for a better understanding of how colours are created and manipulated using different colour models and media.
  • Regardless of whether additive or subtractive colour is to be explored the easiest way to identify the relationship between colours is by using a colour picker.
  • A colour picker is a visual tool that allows the user to select a colour from a colour spectrum or a colour model such as RGB, HSL, or CMYK.
  • Some colour pickers include numeric input fields for entering exact colour values, or a colour palette that includes predefined colours.
  • Colour pickers may show colour relationships in the form of a grid, wheel or in-line with one another.
  • The alternative to colour pickers is to calculate the relationship between colour values mathematically but can be a time-consuming process.
Mixing CMY colours on screen
  • The primary colours in the CMY colour model are:
    • Cyan
    • Magenta
    • Yellow
  • A secondary colour is produced by mixing two primary colours of equal intensity:
    • Magenta + Yellow = Red
    • Yellow + Cyan = Green
    • Cyan + Magenta = blue
  • Mixing secondary colours produces darker versions of their common primary colour:
    • Green + Blue = dark cyan or teal
    • Blue + Red =  dark magenta or purple
    • Red + Green =  dark yellow or olive.
  • Each secondary colour is the complement of one primary colour:
    • Red complements cyan
    • Green complements magenta
    • Blue complements yellow
  • When a primary and its complementary secondary colour are mixed together, the resulting colour is the very dark version of the mixed primary colour:
    • Cyan + Red = a very dark cyan or dark teal
    • Magenta + Green = a very dark magenta or dark purple
    • Yellow + Blue = a very dark yellow or dark olive
    • The exact colour depends on the specific tints of shades of the colours being used and the proportions in which they are mixed.
  • When all the primary colours are mixed in equal intensities, the result is a dark grey or brown colour, but not true black.
  • True black is achieved by using black ink (K) in addition to the CMY colours, creating the CMYK colour model commonly used in printing.
Mixing RYB colours on screen
  • A secondary colour is produced by mixing two primary colours of equal intensity:
    • Red + Yellow = Orange
    • Yellow + Blue = Green
    • Blue + Red = Purple
  • Mixing secondary colours produces darker versions of their common primary colour:
    • Green + Purple = Dark Blue
    • Purple + Orange = Dark Red
    • Orange + Green = Dark Yellow
  • Each secondary colour is the complement of one primary colour:
    • Orange complements blue
    • Green complements red
    • Purple complements yellow
  • When a primary and its complementary secondary colour are mixed together, the resulting colour is the very dark version of the mixed primary colour:
    • Blue + Orange = Dark brown
    • Red + Green = Dark olive
    • Yellow + Purple = Brown
    • The exact colour depends on the specific tints of shades of the colours being used and the proportions in which they are mixed.
  • When all the primary colours are mixed in equal intensities, the result is a dark grey or brown colour, but not true black.
  • True black can be achieved by adding black paint or ink.

Sun

The Sun is the star at the centre of our solar system. It is a giant ball of hot plasma held together by its own gravity.

Here are some basic facts about the Sun:

  • Age: 4.6 Billion Years.
  • Star type: Yellow Dwarf (G2V).
  • Diameter: 1,392,684 km (109 x Earth).
  • Mass: 333,060 x Earth.
  • Surface temperature: 5500 °C.
  • Internal temperature: 15 million °C.
  • Composition: Hydrogen (72%), and helium (26%).
  • Energy generation: Thermo-nuclear fusion using hydrogen as fuel.
  • Energy production: Equal to 100 billion tons of dynamite per second.
  • Energy output: Electromagnetic radiation.
  • Electromagnetic radiation emitted by the Sun = Solar energy or solar radiation.
  • Visible solar radiation = Sunlight.
  • Sunlight: Takes 8 minutes to reach Earth (150 million km).
  • Wavelengths between red and violet are visible to the human eye.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • The Sun is the star at the centre of our solar system. It is a giant ball of hot plasma held together by its own gravity.
  • Here are some basic facts about the Sun:
    • Age: 4.6 Billion Years.
    • Star type: Yellow Dwarf (G2V).
    • Diameter: 1,392,684 km (109 x Earth).
    • Mass: 333,060 x Earth.
    • Surface temperature: 5500 °C.
    • Internal temperature: 15 million °C.
    • Composition: Hydrogen (72%), and helium (26%).

Sun, observer and anti-solar point

The exact position at which an atmospheric rainbow will appear in the sky can be anticipated by imagining a straight line that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.

  • The imaginary line that joins the Sun, observer and the centre of the rainbow is called the rainbow axis.
  • The point on the rainbow axis around which a rainbow appears is called the anti-solar point. The centre of a rainbow coincides with the anti-solar point.
  • Stand with the Sun on your back and look at the ground on a sunny day, the shadow of your head marks the point called the antisolar point, it is 180° away from the Sun.
  • The red arc of a primary bow forms at an angle of 42.40 from the rainbow axis.
  • Seen from an observer’s point of view, the angle outwards from the rainbow axis to the coloured arcs is called the viewing angle.
  • In diagrams, the same angle between the axis and a line extended from an observer’s eyes to the arcs of a rainbow is called the angular distance.
  • With the Sun behind you, spread out your arms to either side or up and down to get a sense of where a rainbow should appear if the conditions are right.
  • Unless seen from the air, the centre of a rainbow and the anti-solar point will always be below the horizon.
  • The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
  • To see a secondary rainbow look for the primary bow first – it has red on the outside. The secondary bow will be a bit larger with violet on the outside at an angle of 53.40 and red on the inside.

Sunlight

Sunlight, also known as daylight or visible light, refers to the portion of electromagnetic radiation emitted by the Sun that is detectable by the human eye. It is one form of the broad range of electromagnetic radiation produced by the Sun. Our eyes are particularly sensitive to this specific range of wavelengths, enabling us to perceive the Sun and the world around us.

  • Sunlight is only one form of electromagnetic radiation emitted by the Sun.
  • Sunlight is only a very small part of the electromagnetic spectrum.
  • Sunlight is the form of electromagnetic radiation that our eyes are sensitive to.
  • Other types of electromagnetic radiation that we are sensitive to, but cannot see, are infrared radiation that we feel as heat and ultraviolet radiation that causes sunburn.

Sunlight

Sunlight, also known as daylight or visible light, refers to the portion of electromagnetic radiation emitted by the Sun that is detectable by the human eye. It is one form of the broad range of electromagnetic radiation produced by the Sun. Our eyes are particularly sensitive to this specific range of wavelengths, enabling us to perceive the Sun and the world around us.

  • Sunlight is only one form of electromagnetic radiation emitted by the Sun.
  • Sunlight is a form of electromagnetic radiation that our eyes are sensitive to.
  • We are sensitive to other types of electromagnetic radiation, such as infrared radiation that we feel as heat, and ultraviolet radiation which can cause sunburn but is invisible to us.
  • The electromagnetic spectrum includes all possible wavelengths of electromagnetic radiation, ranging from low-energy radio waves through visible light up to high-energy gamma rays.
  • Sunlight is a tiny portion of a much larger spectrum called the electromagnetic spectrum.
  • This spectrum encompasses all possible wavelengths of electromagnetic radiation, ranging from low-energy radio waves to high-energy gamma rays. Visible light, the portion we perceive as sunlight, occupies a small band within this spectrum.
  • The human eye is sensitive to this specific range of wavelengths, allowing us to distinguish colours from red to violet.
ABOUT SUNLIGHT & NUCLEAR FUSION

The Sun generates electromagnetic waves primarily through nuclear fusion. Here’s a step-by-step explanation:

Nuclear fusion
  • At the Sun’s core, extremely high temperatures and pressure allow for the fusion of hydrogen nuclei (protons) into helium.
    • This process is also known as thermonuclear fusion.
    • During this reaction, a small portion of the mass of the hydrogen atoms is converted into energy according to Einstein’s mass-energy equivalence principle (E=mc^2).
Photon production
  • The energy produced by nuclear fusion is initially in the form of high-energy gamma photons.
Photon’s journey to the surface
  • Gamma photons then embark on a zig-zag journey to the surface of the Sun, being absorbed and re-emitted by atoms in the Sun’s interior and gradually losing energy in the process.
Surface emission
  • Once photons reach the Sun’s surface (photosphere), they escape and radiate into space. While the majority of this energy is in the form of visible light, it also emits significant amounts of energy in the ultraviolet and infrared parts of the spectrum, as well as smaller amounts in the X-ray, gamma ray, and radio wave parts of the spectrum.
Solar radiation
  • The emitted electromagnetic waves, known collectively as solar radiation or sunlight, then travel through space and can interact with objects they encounter, such as planets. For Earth, these interactions provide light and heat essential to life.
Magnetic field
  • It’s also worth noting that the Sun’s magnetic field can contribute to the generation of some forms of electromagnetic radiation, like solar flares or coronal mass ejections, which can emit radio waves and X-rays.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Sunlight & nuclear fusion

About sunlight & nuclear fusion

The Sun generates electromagnetic waves primarily through nuclear fusion. Here’s a step-by-step explanation:

Nuclear fusion
  • At the Sun’s core, extremely high temperatures and pressure allow for the fusion of hydrogen nuclei (protons) into helium.
    • This process is also known as thermonuclear fusion.
    • During this reaction, a small portion of the mass of the hydrogen atoms is converted into energy according to Einstein’s mass-energy equivalence principle (E=mc^2).
Photon production
  • The energy produced by nuclear fusion is initially in the form of high-energy gamma photons.
Photon’s journey to the surface
  • Gamma photons then embark on a zig-zag journey to the surface of the Sun, being absorbed and re-emitted by atoms in the Sun’s interior and gradually losing energy in the process.
Surface emission
  • Once photons reach the Sun’s surface (photosphere), they escape and radiate into space. While the majority of this energy is in the form of visible light, it also emits significant amounts of energy in the ultraviolet and infrared parts of the spectrum, as well as smaller amounts in the X-ray, gamma ray, and radio wave parts of the spectrum.
Solar radiation
  • The emitted electromagnetic waves, known collectively as solar radiation or sunlight, then travel through space and can interact with objects they encounter, such as planets. For Earth, these interactions provide light and heat essential to life.
Magnetic field
  • It’s also worth noting that the Sun’s magnetic field can contribute to the generation of some forms of electromagnetic radiation, like solar flares or coronal mass ejections, which can emit radio waves and X-rays.

Supernumerary rainbows

Supernumerary rainbows are faint bows that appear just inside a primary rainbow. Several supernumerary rainbows can appear at the same time with a small gap between each one.

  • The word supernumerary means additional to the usual number. The first supernumerary rainbow forms near the violet edge of the primary bow and is the sharpest. Each subsequent supernumerary bow is a little fainter.
  • Supernumerary bows often look like fringes of pastel colours and can change in size, intensity and position from moment to moment.
  • Supernumerary rainbows are clearest when raindrops are small and of equal size.
  • On rare occasions, supernumerary rainbows can be seen on the outside of a secondary rainbow.
  • Supernumerary rainbows are produced by water droplets with a diameter of around 1 mm or less. The smaller the droplets, the broader the supernumerary bands become, and the less saturated are their colours.
  • Supernumerary bows result from the wave-like nature of light and are caused by interference between the waves that contribute towards the main bow. In some places, the waves amplify each other, and in others, they cancel each other out.
  • The theory is that rays of a similar wavelength have slightly different distances to travel through misshapen droplets affected by turbulence, and this causes them to get slightly out of phase with one another. When rays are in phase, they reinforce one another, but when they get out of phase they produce an interference pattern that appears inside the primary bow.