Dictionary tag: S

Substance

A substance is a type of matter with uniform properties throughout. This means that a substance sample 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 with the same structure and composition.
  • Chemical bonds hold these molecules together. Substances cannot be separated into their 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.

Strong nuclear force

The strong nuclear force is one of the four fundamental forces in nature. The other forces are the electromagnetic force, the weak nuclear force and gravity.

  • The strong nuclear force is the strongest of the four fundamental forces of nature but only acts over very small distances, about the size of an atom’s nucleus. This short-range force is about 100 times stronger than the electromagnetic force, 106 times stronger than the weak nuclear force, and 1038 times stronger than gravity.
  • The strong nuclear force is the fundamental force that binds matter together and is responsible for holding together protons and neutrons which are the subatomic particles within the atomic nucleus.
  • The strong nuclear force counteracts the electrical repulsion between protons, which would otherwise push the positively charged protons apart.
  • The strong nuclear force plays a crucial role in nuclear reactions, allowing the release of tremendous energy in processes like nuclear power generation and nuclear weapons.

Stellar light

Stellar light is the term used to describe the electromagnetic radiation emitted by stars, primarily due to the nuclear fusion of hydrogen atoms occurring within their cores.

  • Unlike traditional sources of light on Earth, stars ignite with a far more powerful process – nuclear fusion.
  • Deep within their incredibly dense and hot cores, immense pressure and temperatures fuel nuclear fusion.
  • This process forces hydrogen atoms to merge into heavier elements, primarily helium, releasing tremendous energy.
  • A fraction of this energy escapes the star as the radiant light we call sunlight and starlight.

Standard Model

The Standard Model is a quantum field theory, which means it uses the principles of quantum mechanics to describe the behaviour of matter and energy at the atomic and subatomic levels.

  • The Standard Model is based on two fundamental theories:
    • Quantum mechanics describes the physical properties of nature as interactions between fields of energy at the scale of atoms and subatomic particles. It is the foundation of all quantum physics.
  • Special relativity is a theory of space and time developed by Albert Einstein in 1905. It states that:
    • The laws of physics are invariant (i.e., identical) in all inertial frames of reference.
    • The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source or observer.

Specular reflection

Objects with smooth surfaces produce specular (mirror-like) reflections because light reflects off their surfaces at consistent angles.

  • All objects obey the law of reflection on a microscopic level.
  • If the irregularities on the surface of an object are smaller than the wavelengths of incident light then reflected light travels away from the surface at consistent angles.
  • When an observer looks at specular reflections (regular reflections), they see mirror-like images on the surface of an object.
About diffuse reflection
  • If the irregularities on the surface of an object are larger than the wavelengths of the incident light, light reflects in all directions and produces diffuse reflections.
  • A diffuse reflection is easily distinguished from the mirror-like qualities of a specular reflection.
  • Diffuse reflection is responsible for the way we perceive the colours and textures of objects.

Spectrum

The visible spectrum refers to the range of colours the human eye can perceive, typically seen when light is refracted through a prism, water droplets, or similar mediums. It spans wavelengths from approximately 380 nm (violet) to 700 nm (red), with each wavelength corresponding to a specific colour, from violet through blue, green, yellow, and red.

  • The visible spectrum consists of a continuous distribution of colours, formed by a range of wavelengths rather than distinct, separate bands. While we commonly refer to colours like red, green, and violet, the transitions between them are gradual, with no sharp boundaries.
  • A diagram of the visible spectrum typically displays this continuous range as a linear scale, arranged by wavelength, with red at the longer wavelength end (around 700 nm) and violet at the shorter wavelength end (around 380 nm). This kind of diagram allows us to see the full gradation of colours the human eye can perceive.
  • The visible spectrum is naturally produced when light is refracted through a prism, raindrops, or similar mediums, splitting the light into its component wavelengths. This process of separating light is known as dispersion. The resulting diagram, often called a spectrum, visually represents the distribution of spectral colours as a smooth, elongated band from red to violet, enabling us to observe the gradual transitions between colours.
  • Although the spectrum contains an infinite number of colours due to its continuous nature, most diagrams illustrate a limited number of distinguishable colours between red, orange, yellow, green, blue, indigo, and violet.

Spectral power distribution

The spectral power distribution (SPD) provides a detailed profile of the light emitted or reflected by a source across the visible spectrum, typically represented as a graph where the x-axis shows the wavelength (or frequency) and the y-axis shows the intensity or power at each wavelength.

  • Spectral power distribution is usually measured with a spectroscope. These instruments break down the light into its constituent wavelengths, allowing for precise analysis of the light’s spectral composition. This helps with understanding the exact colour of a light source or how it interacts with materials.
  • SPD is critical in defining colour perception. The way the human eye perceives colour is heavily influenced by the distribution of power at various wavelengths, as different combinations of wavelengths will stimulate the cones in our retinas to varying degrees, resulting in a specific colour experience.
  • SPD helps in identifying and comparing light sources. Different light sources, such as sunlight, LED lamps, or incandescent bulbs, have distinct SPDs. For instance, sunlight has a broad, continuous spectrum, while LEDs or fluorescents often have spikes at certain wavelengths, affecting how we perceive colour under these lights.
  • SPD plays a key role in material appearance. When light reflects off a surface, the spectral power distribution of the reflected light reveals how different wavelengths are absorbed or reflected by the material, influencing the material’s colour and brightness.

Spectral colour model

The spectral colour model represents the range of pure colours that correspond to specific wavelengths of visible light. These colours are called spectral colours because they are not created by mixing other colours but are produced by a single wavelength of light. This model is important because it directly reflects how human vision perceives light that comes from natural sources, like sunlight, which contains a range of wavelengths.

  • The spectral colour model is typically represented as a continuous strip, with red at one end (longest wavelength) and violet at the other (shortest wavelength).
  • Wavelengths and Colour Perception: In the spectral colour model, each wavelength corresponds to a distinct colour, ranging from red (with the longest wavelength, around 700 nanometres) to violet (with the shortest wavelength, around 400 nanometres). The human eye perceives these colours as pure because they are not the result of mixing other wavelengths.
  • Pure Colours: Spectral colours are considered “pure” because they are made up of only one wavelength. This is in contrast to colours produced by mixing light (like in the RGB colour model) or pigments (in the CMY model), where a combination of wavelengths leads to different colours.
  • Applications: The spectral colour model is useful in understanding natural light phenomena like rainbows, where each visible colour represents a different part of the light spectrum. It is also applied in fields like optics to describe how the eye responds to light in a precise, measurable way.

Spacetime

Spacetime combines the three dimensions of space (length, width, height) and the one dimension of time into a single four-dimensional continuum. This continuum is often visualized as a flexible fabric, like a rubber sheet, that can bend and curve in response to mass and energy.

  • Spacetime and light are closely related insofar as the speed of light is constant in all frames of reference. This means that the speed of light is the same for all observers, regardless of the speed and direction in which each observer is moving.
  • This constancy of the speed of light as it travels through spacetime means:
    • The speed of light in a vacuum is 299,792,458 meters per second (m/s). This is believed to be true for all observers.
    • There is no absolute reference frame for space or time, in other words, everything is in motion relative to everything else and so regardless of the place or speed at the moment of measurement, the speed of light always appears the same. As a result,  light travels at the same speed, regardless of whether an observer is moving towards or away from the light source.
    • If the speed of light is a constant then it must be spacetime that curves.
    • The idea that spacetime is curved refers to the idea that if the speed of light is a constant then spacetime must be dynamic.
    • In practice,  the path of light through space is affected by gravity, and gravity causes spacetime to bend. For example, the curvature of spacetime around a massive object, such as a star, will cause light rays to bend. This is known as gravitational lensing.

Sonoluminescence

Sonoluminescence is the emission of light from bubbles undergoing rapid changes in pressure within a liquid when irradiated with sound waves.

  • Bubble formation: Sound waves passing through a liquid create tiny bubbles (cavitation bubbles). These bubbles exist for extremely short periods.
  • Rapid collapse: The sound waves cause the bubbles to expand and then rapidly collapse. During the collapse, the pressure and temperature inside the bubble increase dramatically.
  • Light Emission: The extreme conditions within the collapsing bubble cause the gases inside to become briefly ionized (like a tiny plasma). As these ionized gases return to their normal state, they release energy in the form of a flash of light.

Slow medium

Light travels through different media such as air, glass or water at different speeds. A slow medium is one through which it passes more slowly.

  • Light travels through a vacuum at 299,792 kilometres per second.
  • Light travels through other materials at lower speeds. In some materials, it travels at a speed closer to the speed of light in a vacuum, and in others, it travels much more slowly.
  • The speed of light in air is about 299,702 kilometres per second whilst it travels at approximately 123,889 kilometres per second through diamond.
  • It is useful to know whether a material is fast or slow to predict what will happen when light crosses the boundary between one material and another.
    • As light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
    • As light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.

Secondary rainbow

A secondary rainbow is formed when sunlight undergoes two internal reflections within water droplets, creating an arc with colours reversed from the primary rainbow (violet on the outside, red on the inside). It appears larger and fainter due to light loss during the second reflection and a broader spread of wavelengths.

  • A secondary rainbow appears when sunlight is refracted as it enters raindrops, reflects twice off the inside surface, is refracted again as it escapes back into the air, and then travels towards an observer.
  • A secondary rainbow always appears alongside a primary rainbow and forms a larger arc with the colours reversed.
  • A secondary rainbow has violet on the outside and red on the inside of the bow.
  • When both primary and secondary bows are visible they are often referred to as a double rainbow.
  • A secondary rainbow forms at an angle of between approx. 50.40 to 53.40 to its centre as seen from the point of view of the observer.

Scotopic curve

A scotopic curve is a graphical representation of the sensitivity of the human eye to light under low-light conditions, such as at night or in very dimly lit environments.

  • The scotopic curve resembles a line graph that shows how sensitive the human eye is to light in these low-light conditions. It is an important tool for understanding night vision. The curve illustrates the minimum amount of light needed for the eye to detect different wavelengths (colours) of light.
  • This information is derived from the response of our rod cells, which are more active in low light compared to the cone cells that dominate in bright conditions.
  • Closely related to the scotopic curve is the photopic curve, which represents the sensitivity of the human eye to different wavelengths of light under well-lit conditions. While the scotopic curve peaks at around 498 nanometers (blue-green light), indicating that our eyes are most sensitive to these wavelengths in low light, the photopic curve peaks at around 555 nanometers (green-yellow light) under bright conditions.
  • It is interesting to note that the scotopic and photopic curves use different units to measure light. The scotopic curve uses units related to light intensity per unit area (such as brightness per square degree), whereas the photopic curve uses units similar to overall brightness.

Saturation

Saturation refers to the perceived difference between one colour and another in terms of its purity and vividness.

  • A fully saturated colour appears bright and vibrant because it has a single strong dominant hue.
  • A freshly cut tomato is a good example of a saturated colour with a strong red hue.
  • A saturated colour is a unique spectral colour produced by a single wavelength (or a narrow band of wavelengths) of light.
  • A fully saturated colour (100%) is the purest version of a hue.
  • Unsaturated colours (0-10%) can appear:
    • Misty or milky the nearer they are to white.
    • Dull and washed out as their hue disappears leaving achromatic grey tones.
  • The hue of a vivid colour appears to be at full strength and can leave an after-image of its complementary colour as an observer looks away.

Snell’s Law Calculator

To calculate the angle of refraction of an incident ray entering a raindrop enter:

  1. The refractive index of air for a ray with wavelength 589.29 nm. n1) = 1.000293
  2. The refractive index of water for ray with wavelength 589.29 nm, (n2) = 1.3333
  3. The Angle of incidence of your ray as it strikes the surface. (θi) = angle between the ray and the normal

The angle of refraction (θ2) will appear in the final box.[/vc_column_text][/vc_column]

Snell’s Law Calculator

https://www.omnicalculator.com/physics/snells-law

Sine

In mathematics, the sine is a trigonometric function of an angle.

  • The sine of an acute angle is defined in the context of a right-angle triangle.
  • For any specified angle, it’s sine is the ratio of the length of the side opposite that angle, to the length of the longest side of the triangle (the hypotenuse).
  • The mathematical notation for sine is sin.

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.

 

Scattering

Scattering occurs when light waves interact with particles or irregularities within a medium, causing the light to change direction. This can happen when light encounters obstacles such as atmospheric molecules, dust particles, or surface imperfections.

  • Scattering happens when individual photons or light waves are deflected in different directions, depending on the medium’s composition, particle size, and surface properties.
  • Scattering contributes to various natural phenomena, such as the sky’s blue colour, the whiteness of clouds, and the shimmering of water surfaces.
  • Scattering differs from other optical phenomena:
    • Reflection: Light bounces back, as in a mirror.
    • Refraction: Light is bent as it passes through different materials.
    • Diffraction: Light spreads out after encountering an obstacle.
    • Absorption: Light is absorbed by the material and not re-emitted.Scattering differs from other optical phenomena.
  • Scattering can be effectively subdivided into regular scattering and random scattering, each characterized by distinct mechanisms and patterns of light interaction.

 

Speed of light

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.

  • The speed of light is measured in metres per second (m/s).
  • Light travels through a vacuum at 300,000 kilometres per second.
  • The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
  • Light travels through other media at lower speeds.
  • A vacuum is a region of space that contains no matter.
  • Matter is anything that has mass and occupies space by having volume.
  • When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
  • In other contexts empty space is not considered to be a medium because it does not contain matter.

Solar radiation

Solar radiation is the electromagnetic radiation emitted by the sun.

  • Electromagnetic radiation is a type of energy that is commonly known as light. Detached from its source, it is transported by electromagnetic waves (or by their quanta, particles called photons) and propagates through space.
  • Electromagnetic radiation includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
  • Electromagnetic radiation is sometimes called EM radiation or electromagnetic radiant energy (EMR).
  • All forms of electromagnetic radiation can be described in terms of both waves or particles.
  • All forms of electromagnetic radiation travel at 299,792 kilometres per second in a vacuum.

Secondary colour

A secondary colour is created by mixing two primary colours in equal parts within a particular colour model. The colour space can belong to either an additive colour model, which combines different light wavelengths, or a subtractive colour model, which mixes pigments or dyes.

  • In additive colour models such as the RGB colour model, which deals with the effects of mixing coloured light, a secondary colour results from the overlap of the primary colours: red, green, and blue. The secondary colours produced by mixing pairs of primary colours in the RGB model are cyan, magenta, and yellow.
  • In subtractive colour models such as the CMY colour model, which is concerned with mixing dyes and inks, a secondary colour results from the overlap of the primary colours: cyan, magenta, and yellow. The secondary colours produced by mixing pairs of primary colours in the CMY model are red, green, and blue.

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.

Spectral power distribution

The spectral power distribution (spectral distribution) of a light or colour stimulus refers to the range, mixture and intensity of wavelengths of light that it produces.

  • A diagram showing the accurate measurement of the spectral power distribution of the light given off (emitted, transmitted, or reflected) by a light source or by a surface provides complete information about that stimulus.
  • The human eye contains three colour receptors (cones), which means that every spectral power distribution is reduced to three sensory quantities that can be described by tristimulus values.
  • Different stimuli can in some cases produce the same colour sensation for an observer. This effect (called metamerism) occurs because each type of cone responds to the distribution of energy across the entire spectral power distribution of a light source.

Sun

The Sun is the star at the centre of our solar system.

  • The energy emitted by the Sun is called electromagnetic radiation or solar radiation.
  • The solar radiation that the human eye is sensitive to is often called sunlight or visible light.
  • The term light is often used to refer to visible light but can also be used to refer to all the different forms of electromagnetic radiation.