Interference

Light interference occurs when two or more light waves interact with one another, resulting in a combination of their amplitudes. The resulting wave may increase or decrease in amplitude.

    • A simple form of interference takes place when two plane waves of the same frequency meet at an angle and combine.
    • Light interference is often observed as interference patterns, such as seen in supernumerary rainbows.
    • Interference patterns are produced when the energy of waves combines constructively or destructively. For example, waves on a pond can create interference patterns.
      • Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The resulting wave is the sum of the amplitudes of the original waves.
      • Destructive interference occurs when the trough of one wave meets the crest of another wave. The resulting wave is the difference between the amplitudes of the original waves.
  • In the field of optics, a classical wave model based on the Huygens–Fresnel principle is commonly used to explain optical interference.
  • In the field of quantum mechanics, another model called the Path Integral formulation, developed by Richard Feynman, is used to explain light interference.
About two-point source interference patterns

A two-point source interference pattern is a pattern that results from the superposition of two waves emanating from two different sources that are in phase with each other. The pattern has the following features:

  1. Interference fringes: The interference pattern consists of bright and dark fringes, known as interference fringes. The bright fringes represent constructive interference, where the peaks of the two waves coincide, while the dark fringes represent destructive interference, where the peak of one wave coincides with the trough of the other.
  2. Equidistant fringes: The interference fringes are equidistant from each other, with the distance between adjacent bright fringes (or adjacent dark fringes) being the same.
  3. Central maximum: The central region of the pattern is the brightest, and corresponds to the point where the two waves are in phase and reinforce each other most strongly.
  4. Symmetry: The pattern is symmetric about the midpoint between the two sources.
  5. Wavelength dependence: The spacing between the fringes depends on the wavelength of the light, with shorter wavelengths producing fringes that are closer together than longer wavelengths.
  6. Amplitude dependence: The spacing between the fringes also depends on the amplitude of the waves, with waves of higher amplitude producing fringes that are further apart than waves of lower amplitude.
  7. Angle dependence: The angle between the two sources and the position of the observer relative to the sources can affect the interference pattern, causing it to shift or distort.

HSB colour model & colour brightness

About the HSB colour model and colour brightness

The HSB colour model is an additive colour model used to mix light. Subtractive colour models are used to mix pigments and inks.

  • The only difference between the RGB and HSB colour models is the way colours are represented in terms of colour notation and dealt with in software and apps.
  • Both the HSB and RGB colour model deal with how to mix red, green and blue light to produce other colours.
  • HSB is popular because it provides an intuitive way to select and adjust colours when using applications such as Adobe Creative Cloud for design, photography or web development.
  • The HSB colour model can be used to describe any colour on a TV, computer or phone.

In the HSB colour model:

  • Hue refers to the perceived difference between one colour and another and accounts for colour names such as red, yellow, green or blue.
    • Hue can be measured as a location on a colour wheel and expressed in degrees between 00 and 2590.
  • Saturation refers to the perceived difference between one colour and another in terms of purity.
    • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out until all colour disappears leaving only a monochromatic grey tone (0%).
    • A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths of light.
    • On HSB colour wheels, saturation is usually shown to increase from the centre to the circumference.
  • Brightness (colour brightness) refers to the difference between a hue that appears bold and vivid at maximum brightness (100%) and then appears progressively darker in tone until it appears black at minimum brightness(0%).
  • Colour brightness is often apparent in the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.

HSB colour model & saturation

About the HSB colour model and saturation

The HSB colour model is an additive colour model used to mix light. Subtractive colour models are used to mix pigments and inks.

  • The only difference between the RGB and HSB colour models is the way colours are represented in terms of colour notation and dealt with in software and apps.
  • Both the HSB and RGB colour models deal with how to mix red, green and blue light to produce other colours.
  • HSB is popular because it provides an intuitive way to select and adjust colours when using applications such as Adobe Creative Cloud for design, photography or web development.
  • The HSB colour model can be used to describe any colour on a TV, computer or phone.

In the HSB colour model:

  • Hue refers to the perceived difference between one colour and another and accounts for colour names such as red, yellow, green or blue.
    • Hue can be measured as a location on a colour wheel and expressed in degrees between 00 and 2590.
  • Saturation refers to the perceived difference between one colour and another in terms of purity.
    • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out until all colour disappears leaving only a monochromatic grey tone (0%).
    • A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths of light.
    • On HSB colour wheels, saturation is usually shown to increase from the centre to the circumference.
  • Brightness (colour brightness) refers to the difference between a hue that appears bold and vivid at maximum brightness (100%) and then appears progressively darker in tone until it appears black at minimum brightness(0%).
  • Colour brightness is often apparent in the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.

Interneuron

Interneurons are a type of neuron found in the nervous system of animals, including humans, which play a role in processing and communicating information.

  • Interneurons can be classified into different types based on their functions, such as local circuit interneurons and relay interneurons.
    • Local circuit interneurons have short axons and form circuits with nearby neurons to analyze and process information locally.
    • Relay interneurons have long axons and connect circuits of neurons in different regions of the central nervous system, enabling communication and integration of information.
  • Interneurons can be further classified into subclasses based on their neurotransmitter type, morphology, and connectivity.
  • Interneurons serve as nodes within neural circuits, enabling communication and integration of sensory and motor information between the peripheral nervous system and the central nervous system.
  • The interaction between neurons and interneurons is essential for the proper functioning of the nervous system, enabling the brain to perform complex processes such as perception, cognition, and motor control.
About interneurons and the human eye
  • There are four types of interneurons in the human eye: amacrine cells, bipolar cells, horizontal cells and Müller cells.
  • Interneurons in the human eye form a complex network of interconnections between photoreceptor cells (i.e., rod and cone cells) and retinal ganglion cells.
  • Rod and cone cells are the photoreceptor cells in the human retina that respond to light.
  • Ganglion cells are the retinal neurons that receive and integrate visual information from photoreceptor cells and then transmit it to the brain via the optic nerve.
  • The complex network of interneurons in the human eye plays an important role in the processing and integration of visual information before it is transmitted to the brain.
  • This network is also responsible for a variety of visual functions, including spatial filtering, contrast enhancement, and colour opponent processing.

Interneurons

Rod and cone photoreceptors within the retina of the human eye encode light into electrical signals that are transmitted via a complex network of interneurons to ganglion cells, which then forward visual information via the optic nerve to the brain.

About interneurons
  • Interneurons form nodes within the neural circuits, enabling communication between sensory or motor neurons within the central nervous system.
  • Visual processing in the retina of the human eye depends on coordinated signalling by interneurons.
  • Interneurons are sometimes referred to as local interneurons and relay interneuron.
    • Local interneurons have short axons and form circuits with nearby neurons to analyse small pieces of information.
    • Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions.
  • The interaction between interneurons allows the brain to perform complex functions such as sense-making.
References
  • https://en.wikipedia.org/wiki/Interneuron
  • https://en.wikipedia.org/wiki/Central_nervous_system

Ganglion cells

Ganglion cells

Retinal ganglion cells are located near the boundary between the retina and the central chamber containing vitreous humour. They collect and process all the visual information gathered directly or indirectly from the forty-something types of rod, cone, bipolar, horizontal and amacrine cells and, once finished, transmit it via their axons towards higher visual centres within the brain.

The axons of ganglion cells form into the fibres of the optic nerve that synapse at the other end on the lateral geniculate nucleus. Axons take the form of long slender fibre-like projections of the cell body and typically conduct electrical impulses, often called action potentials, away from a neuron.

A single ganglion cell communicates with as few as five photoreceptors in the fovea at the centre of the macula. This produces images containing the maximum possible resolution of detail. At the extreme periphery of the retina, a single ganglion cell receives information from many thousands of photoreceptors.

Around twenty distinguishable functional types of ganglion cells resolve the information received from 120 million rods and cones into one million parallel streams of information about the world surveyed by a human observer in real-time throughout every day of their lives. They function to complete the construction of the foundations of visual experience by the retina, ordering the eyes response to light into the fundamental building blocks of vision. Ganglion cells do the groundwork that enables retinal encodings to ultimately converge into a unified representation of the visual world.

Ganglion cells not only deal with colour information streaming in from rod and cone cells but also with the deductions, inferences, anticipatory functions and modifications suggested by bipolar, amacrine and horizontal cells. Their challenge, therefore, is to enable all this data to converge and to assemble it into high fidelity, redundancy-free, compressed and coded form that can continue to be handled within the available bandwidth and so the data-carrying capacity of the optic nerve.

It is not hard to imagine the kind of challenges they must deal with:

  • Information must feed into and support the distinct attributes of visual perception and be available to be resolved within the composition of our immediately present visual impressions whenever needed.
  • Information must correspond with the outstanding discriminatory capacities that enable the visual system to operate a palette that can include millions of perceivable variations in colour.
  • Information about the outside world must be able to be automatically cross-referenced, highly detailed, specifically relevant, spatial and temporally sequenced and available on demand.
  • Information must be subjectively orientated in a way that it is locked at an impeccable level of accurate detail to even our most insane intentions as we leap from rock to rock across a swollen river or dive from an aircraft wearing only a wingsuit and negotiate the topography of a mountainous landscape speeding past at 260km per hour.

It is now known that efficient transmission of colour information is achieved by a transformation of the initial three trivariant colour mechanisms of rods and cones into one achromatic and two chromatic channels. But another processing stage has now been recognised that dynamically readjusts the eye’s trivariant responses to meet criteria of efficient colour information management and to provide us with all the necessary contextualising details as we survey the world around us. Discussion of opponent-processing is dealt with in the next article!

Horizontal cells

Horizontal cells

Horizontal cells are connected to rod and cone cells by synapses and are classed as laterally interconnecting neurons.

Horizontal cells help to integrate and regulate information received from photoreceptor cells, cleaning up and globally adjusting signals passing through bipolar cells towards the regions containing ganglion cells.

An important function of horizontal cells is enabling the eye to adjust to both bright and dim light conditions. They achieve this by providing feedback to rod and cone photoreceptors about the average level of illumination falling onto specific regions of the retina.

If a scene contains objects that are much brighter than others, then horizontal cells are believed to prevent signals representing the brightest objects from dazzling the retina and degrading the overall quality of information.

Caption

The Neuronal Organization of the Retina Richard H. Masland
https://www.cell.com/neuron/fulltext/S0896-6273(12)00883-5?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0896627312008835%3Fshowall%3Dtrue

Fovea centralis

Fovea centralis

The entire surface of the retina contains nerve cells, but there is a small portion with a diameter of approximately 0.25 mm at the centre of the macula called the fovea centralis where the concentration of cones is greatest. This region is the optimal location for the formation of image detail. The eyes constantly rotate in their sockets to focus images of objects of interest as precisely as possible at this location.

Intensity

Intensity measures the amount of light energy passing through a unit area perpendicular to the direction of light propagation.

  • Intensity measures the energy carried by a light wave or stream of photons.
  • When light is modelled as a wave, intensity is proportional to the square of the amplitude.
  • When light is modelled as a particle, intensity is proportional to the number of photons present at any given point in time.
  • The intensity of light falls off as the inverse square of the distance from a point light source increases.
  • Light intensity at any given distance from a light source is directly related to the power of the light source and the distance from the source.
  • The power of a light source describes the rate at which energy is emitted and is measured in watts.
  • The intensity of light is measured in watts per square meter (W/m²) and is also commonly expressed in lux (lx).

Impact parameter

The term impact parameter refers to a scale used on a ray-tracing diagram to measure the point at which incident rays strike the surface of a raindrop. Rays are given a value between 0.0 and 1.0 depending upon their point of impact.

  • For a primary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the upper-most point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
  • For a secondary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the lowest point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
  • An impact parameter is useful because it allows the relationship between equidistant incident rays, the angle at which they strike the surface and their angle of refraction to be plotted.

Geometric raindrops

An idealised raindrop forms a geometrically perfect sphere. Although such a form is one in a million in real-life,  simplified geometrical raindrops help to make sense of rainbows and reveal general rules governing why they appear.

The insights that can be gained from exploring the geometry of raindrops apply to every rainbow, whilst the rainbows we come across in everyday life demonstrate that each individual case is unique.

Don’t forget that the idea of light rays is also a way to simplify the behaviour of light:

  • The idea that light is made up of rays is so commonplace when describing and explaining rainbows that it is easily taken for granted.
  • The idea of light rays is useful when trying to model how light and raindrops produce the rainbow effects seen by an observer.
  • Light rays don’t exist in the sense that the term accurately describes a physical property of light. More accurate descriptions use terms like photons or waves.
Basics of raindrop geometry
  • A line drawing of a spherical raindrop is the starting point for exploring how raindrops produce rainbows.
  • The easiest way to represent a raindrop is as a cross-section that cuts it in half through the middle.
  • A dot or small circle can be used to mark the centre whilst the larger circle marks the circumference.
  • Marking the centre makes it easy to add lines that show the radius and diameter.
  • Marking the centre also makes it easy to add lines that are normal to the circumference.
  • A normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
  • A normal is used in a diagram to connect the centre with a point where a ray strikes the circumference.
  • The diameter of a circle is a line that passes through its centre and is drawn from the circumference on one side to the other.
  • The radius of a circle is a line from the centre to any point on the circumference.
  • The horizontal axis of a raindrop is a line drawn through its centre and parallel to incident light. The vertical axis intersects the horizontal at 900 and also passes through the centre point.
  • The angle at which incident light strikes the surface of a raindrop can be calculated by drawing a line that shows where an incident ray strikes a droplet and then drawing the normal. The angle of incidence is measured between them.
  • The path of light as it strikes the surface and changes direction as it is refracted at the boundary between air and water can be calculated using the Law of Refraction (Snell’s law).
  • When light is refracted as it enters a droplet it bends towards the normal.
  • The law of reflection can be used to calculate the change of direction each time light reflects off the inside surface of the raindrop.
  • When light exits a raindrop the angle of refraction is the same as when it entered but this time bends away from the normal.

Fog bows, dew bows and more

There are many optical effects similar to rainbows.

  • A fog bow is a similar phenomenon to a rainbow. As its name suggests, it is associated with fog rather than rain. Because of the very small size of water droplets that cause fog, a fog bow has only very weak colours.
  • A dew bow can form where dewdrops reflect and disperse sunlight. Dew bows can sometimes be seen on fields in the early morning when the temperature drops below the dew point during the night, moisture in the air condenses, falls to the ground, and covers cobwebs.
  • A moon bow is produced by moonlight rather than sunlight but appears for the same reasons. Moon bows are often too faint to excite the colour receptors (cone cells) of a human eye but can appear in photographs taken at night with a long exposure.
  • A twinned rainbow is produced when two rain showers with different sized raindrops overlap one another. Each rainbow has red on the outside and violet on the inside. The two bows often intersect at one end.
  • A reflection rainbow is produced when strong sunlight reflects off a large lake or the ocean before striking a curtain of rain. The conditions must be ideal if the reflecting water is to act as a mirror. A reflected rainbow appears to be similar to a primary bow but has a higher arc. Don’t get confused between a reflection rainbow that appears in the sky and a rainbow reflected in water.
  • A glory is a circle of bright white light that appears around the anti-solar point.
  • A halo is a circle of bright multicoloured light caused by ice crystals that appears around the Sun or the Moon.
  • A monochrome rainbow only occurs when the Sun is on the horizon. When an observer sees a sunrise or sunset, light is travelling horizontally through the atmosphere for several hundred kilometres. In the process, atmospheric conditions cause all but the longest wavelengths to scatter so the Sun appears to be a diffuse orange/red oval. Because all other wavelengths are absent from a monochrome rainbow, the whole scene may appear to be tinged with a fire-like glow.

Invisible dimensions of rainbows

A typical atmospheric rainbow includes six bands of colour from red to violet but there are other bands of light present that don’t produce the experience of colour for human observers.

  • It is useful to remember that:
    • Each band of wavelengths within the electromagnetism spectrum (taken as a whole) is composed of photons that produce different kinds of light.
    • Remember that light can be used to mean visible light but can also be used to refer to other areas of the electromagnetism spectrum invisible to the human eye.
    • Each band of wavelengths represents a different form of radiant energy with distinct properties.
    • The idea of bands of wavelengths is adopted for convenience sake and is a widely understood convention. The entire electromagnetic spectrum is, in practice, composed of a smooth and continuous range of wavelengths (frequencies, energies).
  • Radio waves, at the end of the electromagnetic spectrum with the longest wavelengths and the least energy, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
  • Microwaves have shorter wavelengths than radio waves, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
  • Longer microwaves (waves with similar lengths to radio waves) pass through the Earth’s atmosphere more easily than the shorter wavelengths nearer the visible parts of spectrum.
  • Infra-red is the band closest to visible light but has longer wavelengths. Infra-red radiation can penetrate Earth’s atmosphere but is absorbed by water and carbon dioxide. Infra-red light doesn’t register as a colour to the human eye.
  • The human eye responds more strongly to some bands of visible light between red and violet than others.
  • Ultra-violet light contains shorter wavelengths than visible light, can penetrate Earth’s atmosphere but is absorbed by ozone. Ultra-violet light doesn’t register as a colour to the human eye.
  • Radio, microwaves, infra-red, ultra-violet are all types of non-ionizing radiation, meaning they don’t have enough energy to knock electrons off atoms. Some cause more damage to living cells than others.
  • The Earth’s atmosphere is opaque to both X-rays or gamma-rays from the ionosphere downwards.
  • X-rays and gamma-rays are both forms of ionising radiation. This means that they are able to remove electrons from atoms to create ions. Ionising radiation can damage living cells.
Remember that:
  • All forms of electromagnetic radiation can be thought of in terms of waves and particles.
  • All forms of light from radio waves to gamma-rays can be thought to propagate as streams of photons.
  • The exact spread of colours seen in a rainbow depends on the complex of wavelengths emitted by the light source and which of those reach an observer.

Hue

Hue is one of the three main properties of colour, alongside saturation and brightness and is described using names such as red, yellow, green or blue.

  • Hue refers to the colour of an object or light source, and is determined by the dominant wavelength of light it emits or reflects.
  • Hue is often used to describe colours in terms of their position on the colour wheel/ Colour wheels are circular diagrams that arranges colours according to their hue.
  • The most commonly used colour wheel is the RGB colour wheel, which includes primary colours of red, green and blue, as well as secondary and tertiary colours.
  • Hues can be warm or cool, depending on their position on the colour wheel. Warm hues are those that include red, orange and yellow, while cool hues include blue, green and purple.
  • The perceived brightness and saturation of a hue can be affected by its surrounding colours, as well as by lighting conditions.
  • The perception of hue is also influenced by cultural and personal associations, as well as context and other environmental factors.
  • In the fields of art, design and visual communication, a good understanding of hue is essential for creating effective and visually appealing colour schemes.
  • In digital imaging and colour reproduction, hue can be adjusted through techniques such as colour correction and colour grading, to achieve the desired colour balance and tone.
About the term hue here at lightcolourvision.org
  • At lightcolourvision.org, we use the term “hue” to refer to the attribute of a colour that distinguishes it from other colours on the colour spectrum.
  • Colour models analyse and describe colours and their attributes in various ways. Some are grounded in the way the human eye perceives colours, others provide mathematical explanations.
  • The RGB colour model is a widely used additive colour model that describes colours in terms of the amounts of red, green, and blue light that are combined to create the colour.
  • In the HSB colour model, hue is one of the three attributes that describe a colour, alongside saturation and brightness.
  • The HSB colour model is commonly used in digital design and is a popular way to describe colours on electronic devices like televisions, computers, and mobile phones.
  • The CMYK colour model is used in print and focuses on the colours created by mixing cyan, magenta, yellow, and black inks on paper. Because the CMYK model doesn’t explicitly use the term “hue,” it is not a primary concern when designing for print.

Gamma correction

Gamma correction, also known as gamma encoding, is a technique used in image processing to adjust the brightness and contrast of an image to produce a more natural and visually appealing appearance.

  • Gamma correction of digital images prevents too much information from being stored about highlights that humans cannot differentiate, and too little information about shadows that require more differentiation to be clearly observed.
  • Gamma correction, adjusts the relationship between the numerical value of a pixel stored in an image file (think JPG or TIFF)  and the brightness of that pixel when viewed on-screen.
  • Gamma correction uses a power function to affect the appearance of an image. A power function is a function with a single term that is the product of a real number, a coefficient, and a variable raised to a fixed real number.
  • In the case of a black and white image, a gamma function affects the highlights (whitest values), mid-tones (greyscale), and shadows (dark areas) differently.
  • The appearance of an image on a digital display is determined by the voltage at each pixel:
    • A computer used to display black and white images for example, translates the numerical values of each pixel in an image file into a voltage that is sent to a monitor. The higher the voltage, the brighter the pixel.
    • The ideal relationship between stored value and appearance is non-linear, so a change in voltage does not necessarily translate directly into a satisfactory change in brightness so far as an observer is concerned.
    • For many TVs and computer displays, doubling the voltage of a particular pixel will not make it appear twice as bright so gamma correction selectively adjusts voltages to improve the final appearance.
    • The menu on most digital displays includes an option to adjust gamma settings.

Index of refraction

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

  • The refractive index of a medium is a numerical value and is represented by the symbol n.
  • Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
  • The refractive index of water is 1.333, meaning that light travels at 2/3 the speed in water compared to a vacuum.
  • If the refractive index of a medium is 1.5, for example, light travels at 2/3 the speed through glass compared to a vacuum.
  • As light undergoes refraction, its wavelength changes, but its frequency remains the same.
  • As light undergoes refraction its frequency remains the same.
  • The energy transported by light is not affected by refraction or the refractive index of a medium, but the intensity of the light can be affected.

Fundamental force

In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions.

The four fundamental forces that account for all the forms of pulling and pushing between things are:

  • Gravitational force: Gravity is the phenomenon that causes things with mass or energy to gravitate towards one another. Planets, stars, galaxies, and even light are all affected by gravity. The effect of gravity on small things like human beings when in the vicinity of something big like a planet is obvious. It is the Moon’s gravity that causes ocean tides on Earth. Gravity accounts for physical objects having weight. Gravity has an infinite range, although its effects become weaker as objects get further away from one another.
  • Weak Nuclear force: In nuclear physics and particle physics, the weak nuclear force explains the interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak nuclear force doesn’t affect electromagnetic radiation.
  • Strong Nuclear force: The strong nuclear force holds matter together. It binds the sub-atomic particles, protons and neutrons, that form the nucleus of an atom. Whilst repulsive electromagnetic forces push them apart, the attractive nuclear force is strong enough to overcome them at short range. The range at work here is measured in femtometres. The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons.
  • Electromagnetic force: The electromagnetic force is the force that occurs between electrically charged particles, such as electrons, and is described as either a positive or negative charge. Objects with opposite charges produce an attractive force between them, while objects with the same charge produce a repulsive force. The electromagnetic force is carried by photons in the form of electric and magnetic fields that propagate at the speed of light.
  • Whenever there is a push-pull interaction between two objects, forces are applied to each of them. When the interaction ceases, the two objects no longer experience the force and their momentum continues uninterrupted.
  • On a macro-scale wherever there is a concentration of stuff, in planets, suns or galaxies for example, that is where massive push-pulls happen.
  • Everything everywhere is in motion. Nothing in the whole Universe is stationary unless its temperature is reduced to absolute zero. But in reality nothing can be cooled to a temperature of exactly absolute zero.
  • Motion applies to everything including objects, bodies, matter, particles, radiation and space-time. We also refer to the motion of images, shapes and boundaries.
  • Motion signifies a change in the position of the elements of a physical system. But an object’s motion (its momentum) stays the same unless a force acts on it.

Internal reflection

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another transparent medium such as air. The light reflects back off the boundary between the two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
  • In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
  • If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
  • When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
  • The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Fast medium

Light travels through different media such as air, glass or water at different speeds.  A fast medium is one through which it passes through more quickly than others.

  • Light travels through a vacuum at 299,792 kilometres per second.
  • Light travels through other media at lower speeds.
  • In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
  • It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
  • so:
  • If 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.
  • If 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.
  • In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

Hexadecimal number

An hexadecimal number (hex number) has a base (radix) of 16 whilst a decimal system of notation has a base of 10.

  • The familiar decimal system of notation uses nine distinct symbols 0 – 9. It then adds columns to the right to denote 10’s, 100’s etc.
  • A hexadecimal system of notation uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F (or a, b, c, d, e, f) to represent values ten to fifteen. Further columns are added on the right to denote 16’s, 256’s etc.
  • A hexadecimal triplet is a six-digit, three-byte hexadecimal system of notation used in programming and software applications (graphic design, web development, photography) to represent colours. The bytes represent the red, then green and then blue components of a colour.
  • Hexadecimal triplets can be used to represent 256 x 256 x 256 different colours.
  • Each byte represents a number in the range 00 to FF in hexadecimal notation (0 to 255 in decimal notation).
  • The hash symbol (#) is used to indicate hex notation.
    • Red = #FF0000
    • Yellow = #FFFF00
    • Green = #00FF00
    • Cyan = 00FFFF
    • Blue = #0000FF
    • Magenta = #FF00FF
  • The sequence of hexadecimal values between 1 and 16 = 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E and F.
  • The sequence of hexadecimal values between 17 and 32 = 10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E and 1F.
  • The sequence then continues to increment the two digits up to 256.