Dictionary tag: R

RYB colour model

The RYB colour model is a subtractive method of colour mixing commonly used in traditional art and painting with opaque pigments.

  • The three primary colours in RYB are red, yellow, and blue.
  • By mixing these primaries in pairs and in the correct proportions, artists can create secondary colours like:
    • Orange (red and yellow),
    • Green (yellow and blue), and
    • Purple (blue and red).
  • Overlapping all three primaries creates darker and more complex hues.
  • While historically significant, the RYB model has been largely replaced by RGB and CMYK in modern digital design and printing due to the incompatibility of RYB with these technologies.

RGB colour wheel

A colour wheel is a diagram based on a circle divided into segments and can be used to explore the effect of mixing adjacent colours.

An RGB colour wheel provides a graphic representation of the RGB colour model.

  • RGB colour wheels have a minimum of three segments or spokes. These are filled with the additive primary colours red, green and blue.
  • Starting with the three primary colours, an RGB colour wheel can demonstrate the effect of mixing adjacent segments to produce progressively more subtle gradations of intermediate hues.
  • An RGB colour wheel is particularly useful when trying to visually identify and specify:
    •  A particular RGB colour
    • The relationship between different RGB colours
    • Find the colour value (code) for an RGB colour.
  • LED light sources producing very narrow bands of wavelengths can be used when demonstrating RGB colour wheels by projecting red, green and blue LEDs onto a neutrally toned surface.
  • The peak wavelength for selected colours might typically be red = 625 nanometres, green = 500 nm and blue = 440 nm.

RGB colour values

RGB colour values refer to the numeric codes used in RGB colour notation to define specific colours in a digital image or on a digital display.

  • Colour values are numbers and characters used by colour models to represent and store colour information in a format recognizable to both computers and humans.
  • Each colour within a colour model is assigned a unique colour value.
  • The notation for RGB colour values (codes) are decimal triplets (base 10) or hexadecimal triplets (base 16).
  • RGB decimal and hexadecimal triplets look like this:
    • R = 255, G = 128, B = 0 (255,128,00) is the decimal notation for orange.
    • #FF8000 is the hexadecimal notation for orange.

RGB colour notation

RGB colour notation refers to the method used by the RGB colour model to identify and store colour values in a format recognizable to both computers and humans.

  • RGB stands for red, green, and blue and is a commonly used colour model in digital imaging and computer graphics.
  • The RGB colour model is an additive colour model, meaning that colours are created by adding different amounts of red, green, and blue light together.
  • RGB colour notation is widely used in web design, graphic design, and other digital media.
  • RGB colour values are expressed as either:
    • Decimal triplets. For example, the colour yellow is represented as 255, 255, 0.
    • Hexadecimal triplets. For example, the colour green is represented as #00FF00.

Rest mass

Rest mass, (also known as invariant mass, intrinsic mass, proper mass, or simply mass) in the case of bound systems, is a fundamental property of a particle or a system of particles that remains constant regardless of the observer’s frame of reference.

  • For a single particle, its invariant mass equals its energy at rest divided by the square of the speed of light.
  • Photons, which have zero rest mass, still possess an invariant mass calculated using their energy and momentum, related by the equation E=pc, where E is energy, p is momentum, and c is the speed of light.
  • Invariant mass is a crucial concept in particle physics because it is conserved in all interactions, including those involving the conversion of mass into energy and vice versa.
  • The conservation of invariant mass stems from the conservation of energy and momentum, fundamental principles in physics.

Refractive index

The refractive index (index of refraction) of a medium measures how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum.

  • Refractive index (or, index of refraction) is a measurement of how much the speed of light is reduced when it passes through a medium compared to the speed of light in a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index can vary with the wavelength of the light being refracted. This phenomenon is called dispersion, and it is what causes white light to split into its constituent colours when it passes through a prism.
  • The refractive index of a material can be affected by various factors such as temperature, pressure, and density.

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the boundary between one transparent medium and another.

  • Light bends towards the normal and slows down when it moves from a fast medium (like air) to a slower medium (like water).
  • Light bends away from the normal and speeds up when it moves from a slow medium (like diamond) to a faster medium (like glass).
  • These phenomena are governed by Snell’s law, which describes the relationship between the angles of incidence and refraction.
  • The refractive index (index of refraction) of a medium indicates how much the speed and direction of light are altered when travelling in or out of a medium.
  • It is calculated by dividing the speed of light in a vacuum by the speed of light in the material.
  • Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media involved.
  • Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the refractive indices.

Reflection

Reflection is the process where light rebounds from a surface into the medium it came from, instead of being absorbed by an opaque material or transmitted through a transparent one.

  • The three laws of reflection are as follows:
    • When light hits a reflective surface, the incoming light, the reflected light, and an imaginary line perpendicular to the surface (called the “normal line”) are all in the same flat area.
    • The angle between the incoming light and the normal line is the same as the angle between the reflected light and the normal line. In other words, light bounces off the surface at the same angle as it came in.
    • The incoming and reflected light are mirror images of each other when looking along the normal line. If you were to fold the flat area along the normal line, the incoming light would line up with the reflected light.

Ray-tracing diagram

A ray-tracing diagram uses drawing conventions and labels to illustrate the path of light rays as they interact with different media, materials, or objects. Ray tracing diagrams help to understand the optical behaviour of the light.

  • Ray-tracing diagrams are used in geometric optics, where light is treated as rays that travel in straight lines and change speed and/or direction as they pass through different transparent media.
  • The purpose of a ray-tracing diagram is to illustrate optical phenomena such as absorption, dispersion, polarization, reflection, refraction, scattering, and transmission.
  • The accuracy of a ray-tracing diagram depends on the quality of the data used to create it, such as the refractive index of the materials and the angles of incidence and reflection.
  • Ray-tracing can be used to design and optimize optical systems, such as lenses and mirrors.

Ray of light

A ray of light (light ray or just ray) is a common term when talking about optics and electromagnetism.

  • A ray of light is a way of imagining, conceptualising and representing the way light moves.
  • The idea of a ray of light is rooted in the observation that light travels in straight lines until it meets an obstacle.
  • It is common sense to think of a narrow beam of light as being composed of parallel arrows or a bundle of rays.
  • The bundle of rays can then be used to trace what happens when light strikes a complex object such as a lens or convex mirror.
  • Single rays are often used to plot the path of a specific wavelength of light and compare it with the path of others.

Ray

A light ray in a diagram is used to show how light moves and changes when it passes through space and different media.

  • Geometric optics uses the concept that light is made up of rays to explain how it behaves as it encounters different materials and media.
  • Imagine a flashlight beam cutting through the night. A light ray in a diagram is a simplified version of that beam, helping us visualize how light travels and changes when it interacts with different media.
  • Light rays are not tangible, they are a theoretical idea used to create a simplified explanation of light.
  • More precise descriptions of light’s characteristics use terms like photons or waves.
  • A light ray is a graphical depiction of a slender light beam moving through either a vacuum or a medium.
  • The closest equivalent to a light ray in real life is a narrow, concentrated light beam generated by a laser.
  • Ray diagrams use straight lines and arrows to demonstrate how light travels through space and transparent media.

Rainbow axis

The rainbow axis is an imaginary straight line that connects the light source, observer and anti-solar point.

  • The centre of a rainbow is always on its axis.
  • The centre of a rainbow always corresponds with the anti-solar point.
  • When drawing a diagram showing the axis of a rainbow, the Sun and anti-solar point, are at opposite ends with the observer between them.
  • From an observer’s point of view, the rainbow axis is an imaginary line that they look along towards the centre of a rainbow.

Radiometry

Radiometry is the study of how light, carried by electromagnetic waves made up of particles called photons, travels through space. It involves measuring and analysing the energy (radiant energy) of these waves and their component particles.

  • Radiometry studies the properties of electromagnetic radiation such as intensity, spectral distribution and polarization, and how light interacts with matter (absorption, reflection, and scattering).
  • Electromagnetic radiation and the electromagnetic energy it transports can be described in terms of waves.
    • Electromagnetic radiation (radiant energy) includes all wavelengths of light from radio waves to gamma rays.
  • Electromagnetic radiation can be described in terms of photons and their properties.
    • Energy: Photons have energy that depends on their frequency or wavelength. Higher-frequency photons have more energy than lower-frequency photons.
    • Number: The number of photons in a given electromagnetic radiation depends on its intensity. Higher-intensity radiation has more photons than lower-intensity radiation.
    • Direction: Photons travel in straight lines, but their direction can be changed by interacting with matter.
    • Polarization: Photons can be polarized, which means that their electric and magnetic fields oscillate in a particular direction.
    • Speed: Photons travel at the speed of light, which is approximately 299,792,458 meters per second in a vacuum.

 

Radiation

Radiation is energy that comes from a source and travels through space at the speed of light.

  • Radiant energy has an electric field, and a magnetic field and may be described in terms of electromagnetic waves or in terms of bundles of photons travelling in a wave-like pattern.
  • Visible light is a form of radiation often described in terms of either electromagnetic waves or photons.
  • Types of radiation with the highest energy include ultraviolet radiation, x-rays, and gamma rays.
  • When x-rays or gamma-rays interact with atoms, they can remove electrons which destabilises them and make them radioactive.
  • Radioactivity is the spontaneous release of energy from an unstable atom as it returns to a stable state.
  • Ionizing Radiation is the energy that comes out of a radioactive atom.

Radiant energy

Radiant energy and electromagnetic radiation are two terms that refer to the same concept. Both refer to the propagation of energy through space in the form of waves. These waves appear as oscillating electric and magnetic fields,  the fundamental feature of electromagnetic radiation.

  • Electromagnetic radiation can be viewed as either electromagnetic waves or a stream of photons. These two perspectives are not mutually exclusive but rather complementary, as explained by the concept of wave-particle duality.
    • Wave Perspective: From this viewpoint, electromagnetic radiation is thought of as waves of oscillating electric and magnetic fields traversing through space. The energy of the radiation is distributed across the wave, with its intensity related to the amplitude and frequency of the wave.
    • Photon Perspective: Electromagnetic radiation can also be thought of as a succession of massless particles known as photons. Each photon carries a discrete quantum of energy, referred to as photon energy, directly related to the radiation’s frequency. This perspective acknowledges the particle-like behaviour of electromagnetic radiation.
    • The concept of wave-particle duality which has been developed in Quantum Field Theory, reconciles these contrasting viewpoints by claiming that electromagnetic radiation exhibits both wave-like and particle-like attributes. This duality has been experimentally verified.

Retinal image

Retinal image

It is the cornea-lens system that determines where light falls on the surface of the retina which results in discernible images.

The images are inverted and obviously very small compared with the world outside that they resolve. The inversion poses no problem. Our brains are very flexible and even when tricked by prisms will always turn the world right-side-up given time. The reduction in size is part of the process by which the fit of the image on the retina determines our field of view.

The images are real in the sense that they are formed by the actual convergence of light rays onto the curved plane of the retina. Only real images of this kind provide the necessary stimulation of rod and cone cells necessary for human perception.

Rods and cones

Rods and cones

Both the photosensitive rods and cones form a regularly spaced mosaic of cells across the entirety of the retina – bar the absence of rods in the fovea centralis. Because there are 100 million rod receptors and 20 million cone receptors in each eye, rods are packed more densely per unit area. The synaptic connections of both rods and cones vary in function in different locations across the retina, reflecting the specialisations of different regions. This, for example, allows the eyes to deal with daylight and darkness and with what we see at the centre and periphery of our field of view.

Rods and cones are easily distinguished by their shape, from which they derive their names, the type of photo pigment they contain and by the distinct patterns of synaptic connections with the other neurons around them.

Neurons (nerve cells) are present throughout the human central and peripheral nervous systems and fall into three main categories: sensory, motor and interneurons. Rods and cones are both sensory neurons. Rods don’t produce as sharp an image as cone cells because they share more connections with other types of neurons. But a rod cell is believed to be sensitive enough to respond to a single photon of light whilst cone cells require tens to hundreds of photons to be activated.

The principal task of rod and cone cells alike is photo-transduction. This refers to the type of sensory transduction that takes place in the visual system. It is the process of photo-transduction that enables pigmented chemicals in the rods and cones to sense light and convert it into electrical signals. Many other types of sensory transduction occur elsewhere within the body enabling touch and hearing for example.

References: Functional Specialization of the Rod and Cone Systems: https://www.ncbi.nlm.nih.gov/books/NBK10850/

Retinal input

Retinal input

Visual input is initially encoded in the retina as a two-dimensional distribution of light intensity, expressed as a function of position, wavelength and time. This retinal image is transferred to the visual cortex where primary sensory cues and, later, inferred attributes, are eventually computed in the process of actualising our perceptions. Parallel processing strategies are employed from the outset to overcome the constraints of the individual ganglion cell’s limited bandwidth and anatomical bottlenecks as data approaches the optic nerves that connect each eye to the visual cortex.

References: DeYoe and Van Essen (1988): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2771435/

Retina

THE RETINA

Human beings see the world in colour because of the way their visual system processes light. The retina contains light-sensitive receptors, rod and cone cells, that respond to light stimuli. It is the variety of wavelengths and intensities of light entering the eyes that produces the impression of colour.

The retina is the innermost, light-sensitive layer of tissue inside our eyes. It forms a sheet of tissue barely 200 micrometres (μm) thick, but its neural networks carry out almost unimaginably complicated feats of image processing.

The physiology of the eye results in a tiny, focused, two-dimensional image of the visual world being projected onto the retina’s surface. Because of the optics of lenses, it appears upside down and the wrong way around. But no worry, sorting that out is child’s play for the human brain! The real challenge is that the photosensitive receptors in the retina must produce precise chemical responses to light and translate every minute detail of the image into electrical impulse ready to be sent to the brain where they produce visual impressions of the world. In a very limited sense, the retina serves a similar function to a photosensitive chip in a camera.

As research continues to reveal ever-increasing amounts of detail about these signalling processes across and beyond the retina, it required new thinking, not only of the retina’s function but also of the mechanisms within the brain that shape these signals into behaviourally useful perceptions.

The retina consists of 60-plus distinct neuron types, each of which plays a specialized role in turning variations in the patterns of wavelengths and intensities of light into visual information. Neurons are electrically excitable nerve cells that collect, process and transmit vast amounts of this information through both chemical and electrical signals. Retinal neurons work together to convert the signals produced by a hundred and twenty million rods and cones and send them along around one million fibres within the optic nerve of each eye to connections with higher brain functions. In this process rods and cones are first responders whilst ganglion cells are the final port of call before information leaves the retina.

There are three principal forms of processing that take place within the retina itself. The first organises the outputs of the rod and cone photoreceptors and begins to compose them into around 12 parallel information streams as they travel through bipolar cells. The second connects these streams to specific types of retinal ganglion cells. The third modulates the information using feedback from horizontal and amacrine cells to create the diverse encodings of the visual world that the retina transmits towards the brain.

As mentioned above, the image of the outside world focused on the retina is upside down and the wrong way around. But the human retina is also inverted in the sense that the light-sensitive rod and cone cells are not located on the surface where the image forms, but instead are embedded inside, where the retina attaches to the fabric of the eyeball. As a result, light striking the retina, passes through layers of other neurons (ganglion, bipolar cells etc.) and blood-carrying capillaries, before reaching the photoreceptors.

The overlying neural fibres do not significantly degrade vision in the inverted retina. The neurons are relatively transparent and accompanying Müller cells act as fibre-optic channels to transport photons directly to photoreceptors. However, some estimates suggest that overall, around 15% of all the light entering the eye is lost en-route to the retina. To counter this, the fovea centralis, at the centre of our field of vision, is free of rods and there are no blood vessels running through it, so optimising the level of detail where we need it most.

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

Reflectance

The reflectance of the surface of a material is its effectiveness in reflecting radiant energy.

  • Reflectance is the fraction of incident electromagnetic power that is reflected at the boundary. Power = energy x time.
  • Reflectance is a component of the response of a material to the electromagnetic properties of light, so a function of its:
    • Angle of incidence
    • Wavelength (or frequency)
    • Polarization
  • Given that reflectance is a directional property, most surfaces can be divided into those that give specular reflection and those that give diffuse reflection.
  • For specular surfaces, such as glass or polished metal, reflectance is nearly zero at all angles except at the angle visible to an observer.
  • For diffuse surfaces, such as matte white paint, reflectance is uniform in all directions so radiation is reflected at all angles equally or near-equally.
  • Most practical objects exhibit a combination of diffuse and specular reflective properties.

ROYGBV

ROYGBV are the initials for the sequence of colours that make up the visible spectrum: red, orange, yellow, green, blue, and violet.

  • The visible spectrum refers to the range of colours visible to the human eye.
  • White light, when passed through a prism, separates into a sequence of individual colours corresponding with ROYGBV which is the range of colours visible to the human eye.
  • White light separates into ROYGBV because different wavelengths of light bend at slightly different angles as they enter and exit the prism.
  • ROYGBV helps us remember the order of these spectral colours starting from the longest wavelength (red) to the shortest (violet).
  • A rainbow spans the continuous range of spectral colours that make up the visible spectrum.
  • The visible spectrum is the small band of wavelengths within the electromagnetic spectrum that corresponds with all the different colours we see in the world.
  • The fact that we see the distinct bands of colour in a rainbow is an artefact of human colour vision.

RGB colour model

RGB colour is an additive colour model in which red, green and blue light is combined to reproduce a wide range of other colours.

  • The primary colours in the RGB colour model are red, green and blue.
  • In the RGB model, different combinations and intensities of red, green, and blue light are mixed to create various colours. When these three colours are combined at full intensity, they produce white light.
  • Additive colour models are concerned with mixing light, not dyes, inks or pigments (these rely on subtractive colour models such as the RYB colour model and the CMY colour model).
  • The RGB colour model works in practice by asking three questions of any colour: how red is it (R), how green is it (G), and how blue is it (B).
  • The RGB model is popular because it can easily produce a comprehensive palette of 1530 vivid hues simply by adjusting the combination and amount of each of the three primaries it contains.

Rainbow colours

Rainbow colours are the colours seen in rainbows and in other situations where visible light separates into its different wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

  • The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
  •  It is the sensitivity of the human eye to this small part of the electromagnetic spectrum that results in our perception of colour.
  • The names of rainbow colours are a matter more closely related to the relationship between perception and language than anything to do with physics or scientific accuracy. While the spectrum of light and the colours we see are both determined by wavelength, it’s our eyes and brains that turn these differences in light into the colours we experience.
  • In the past, rainbows were sometimes portrayed as having seven colours: red, orange, yellow, green, blue, indigo and violet.
  • Modern portrayals of rainbows reduce the number of colours to six spectral colours, ROYGBV.
  • In reality, the colours of a rainbow form a continuous spectrum and there are no clear boundaries between one colour and the next.

 

 

Rainbow

A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction (bending) and dispersion (spreading out) of light in individual droplets and result in the appearance of an arc of spectral colours.

  • Atmospheric rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
  • Waterfalls, lawn sprinklers and other things that produce air-borne water droplets can produce a rainbow.
  • An atmospheric rainbow is formed from countless individual droplets each of which reflects and refracts a tiny coloured image of the Sun towards the observer.
  • As white light passes through water droplets, refraction causes the light to disperse and separate into the different colours seen by an observer.
  • If the sun is behind an observer then the rainbow will appear in front of them.
  • When a rainbow is produced by sunlight, the angles between the sun, each droplet and the observer determine which ones will form part of the rainbow, the colour each droplet will produce and the sequence in which they appear.