## 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.

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

## 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.

## Fundamental Force

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

• Whenever there is a push-pull interaction between two objects, forces are being 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, that is where most of the push-pulls happen.
• Everything everywhere is in motion.
• Nothing in the Universe is stationary unless its temperature is reduced to absolute zero. But nothing can be cooled to a temperature of exactly absolute zero.
• Motion applies to things like objects, bodies, matter, particles, radiation and space-time. We also refer to the motion of images, shapes and boundaries. So motion signifies a change in the position of the elements of a physical system. An object’s motion, and so its momentum stays the same unless a force acts on it.

The four fundamental forces of nature 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.

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

## Force

Forces bind things together and push things apart.

• The push-pull interactions between things are described as the interplay of forces.
• Forces explain how anything interacts with anything else in the whole of the natural world.
• Forces produce motion and can cause an object with mass to change velocity.
• Changes in velocity include causing things to start moving from a state of rest, to accelerate or slow down.
• Quarks and leptons, fundamental particles present in all forms of matter, are bound together by fundamental forces.
• There are four fundamental forces that account for all the forms of pulling and pushing between things in the Universe.

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

## Fovea

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.

## Frequency

The frequency of electromagnetic radiation (light) refers to the number of wave-cycles of an electromagnetic wave that pass a given point in a given amount of time.

• The frequency of a wave should not be confused with the speed at which the wave travels or the distance it travels.
• The term frequency refers to the measurement of the frequency of wave-cycles that pass a given point in a given amount of time.
• Frequency is measured in Hertz (Hz). One Hertz is one wave-cycle per second.
• The wavelength and frequency of light are closely related. the higher the frequency, the shorter the wavelength.
• The amount of energy transported by a light wave increases with the frequency of oscillations and as the length of each oscillation decreases.

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

## Gamut

A colour gamut defines a more specific range of colours from the range of colours identifiable by the human eye.

• The range of perceived colours (visible to a human observer) is always greater than the range that can be reproduced by any digital process, or display device (screen, monitor, projector).
• Digital cameras, scanners, monitors, and printers are all limited to the range of colours they can sense, store and reproduce. A colour gamut is established to make these differences clear and to reconcile the colours that can be used in common between devices.
• A device that can reproduce the entire visible colour space corresponding with human perception is an unrealized goal within the engineering of display devices and printing processes.
• When colours within a colour space cannot be reproduced using a specific colour space or display device those colours are said to be out of gamut.

## Ganglion cell

A retinal ganglion cell is a type of neuron located in the retina of the human eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells.  Retinal ganglion cells transmit image-forming and non-image forming visual information to several regions in the thalamus, hypothalamus and midbrain.

• 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 are like long tails and typically conduct electrical impulses, often called action potentials, away from a neuron. They take the form of long slender fibre-like projections of the cell body.
• 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 a million parallel streams of information about the world surveyed by a human observer throughout every day of their lives. Their functions 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 enable retinal encodings to ultimately converge into a unified representation of the visual world.
• As described above cone cells are attuned to different bands of wavelengths, with peak biases at 560 nm, 530 nm, and 420 nm and are concerned with trivariance – three discernible differences in the overall composition of visible light entering the eye.
• Ganglion cells also play a critical role in trichromacy but the way they function might be thought of as being determined by limitations on bandwidth within the optic nerve.
• Ganglion cells not only deal with colour information streaming in from rod and cone cells in real time 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 a high fidelity, redundancy-free, compressed and coded form that can continue to be handled within the data-carrying capacity of the optic nerve.
• It is not hard to imagine the kind of challenges that have to be dealt 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 colour mechanisms of rods and cones into one achromatic and two opponent chromatic channels. Opponent type processing clearly represents the optimal necessary step to dynamically readjust the eye’s earlier trivariant responses to meet criteria of efficient colour information complete with all the necessary contextualising detail ready for transmission. We can assume it is in response to these demand that every stimulus to the eye can be accurately and objectively defined in both space and time in ways relevant to everyday circumstances.

https://en.wikipedia.org/wiki/Retinal_ganglion_cell>

## Greyscale

Greyscale images are also known as black-and-white or monochrome images and are composed exclusively of shades of grey, varying from black at the weakest intensity to white at the strongest. A greyscale image shows natural colour luminance with hue and saturation removed and so it carries only intensity information.

• Greyscale images have many shades of grey in between black and white.
• Greyscale images are distinct from one-bit bi-tonal black-and-white images which, in the context of computer imaging, are images with only two colours: black and white.
• Greyscale images can be the result of measuring the intensity of light at each pixel against a selected wavelength or a weighted combination of wavelengths. In this case, the selected wavelengths can be from anywhere within the electromagnetic spectrum (e.g. infrared, visible light, ultraviolet.).
• A colourimetric (or more specifically photometric) grayscale image is an image that has a defined greyscale colourspace, which maps the stored numeric sample values to the achromatic channel of a standard colourspace, which itself, is based on measured properties of human vision.

## Hertz (Hz)

The hertz (symbol: Hz) is a unit used to measure the frequency of electromagnetic waves.

• Hertz are used when measuring the frequency of wave-cycles of electromagnetic waves.
• One hertz is defined as one cycle per second.
• Hertz measure the number of oscillations of the perpendicular electric and magnetic fields of electromagnetic radiation per second.
• 1 Hertz (Hz) = 1 cycle per second
1 Kilohertz (kHz) = 1,000 (thousand) cycles per second
1 Megahertz (MHz) = 1,000,000 (million) cycles per second
1 Gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
1 Terahertz (THz) = 1,000,000,000,000 (trillion )cycles per second

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

## Hertz (Hz)

The hertz (symbol: Hz) is a unit used to measure the frequency of electromagnetic waves.

• Hertz are used when measuring the frequency of wave-cycles of electromagnetic waves.
• One hertz is defined as one cycle per second.
• Hertz measure the number of oscillations of the perpendicular electric and magnetic fields of electromagnetic radiation per second.
• 1 Hertz (Hz) = 1 cycle per second
1 Kilohertz (kHz) = 1,000 (thousand) cycles per second
1 Megahertz (MHz) = 1,000,000 (million) cycles per second
1 Gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
1 Terahertz (THz) = 1,000,000,000,000 (trillion )cycles per second

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.
• 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.

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.

## Horizontal cell

Retinal horizontal cells are laterally interconnecting neurons located in the retina of the human eye.

• Horizontal cells are connected to the rod and cone cells by synapses, so horizontal cells are called laterally interconnecting neurons.
• Horizontal cells help to integrate and regulate of photoreceptor cells, cleaning up and globally adjusting signals passing through bipolar cells towards the region 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 quality of information.

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

## HSB colour model

HSB is a popular colour model that provides an intuitive way to select and adjust colours in software applications used for graphic design, web development and photography.

• In the implementation of HSB used in Adobe Illustrator CC, brightness and saturation are used to alter the intensity of a hue. In this way:
• H=00, S=100%, B= 100% produces a fully saturated primary red hue with maximum intensity.
• H=00, S=100%, B= 50% produces a darker shade of a fully saturated primary red hue.
• H=00, S=50%, B= 100% produces a lighter tint and thus a partially de-saturated primary red hue.
• Selecting an RGB swatch 255,0,0 (#FF0000) as a stroke or fill for a selected object.
• Double-clicking the swatch for the selected object, selecting HSB in the Color panel menu and reducing saturation for tints or reducing brightness for shades.
• Or selecting the Color Guide panel with an object selected and selecting
• The brightness of a hue is reduced on a display device by reducing the amplitude of the RGB colours produced by each pixel.
• The brightness of a hue is reduced on a digital printer by increasing the amount of black mixed with red ink.

## HSB colour values

Definition

HSB colour values (codes) are decimal triplet used in software applications to represent colour.

HSB refers to hue, saturation and brightness.

Values applied to each of these three parameters can be used to specify any colour within the RGB colour models used by display devices and printers.

Explanation

In the implementation of the HSB colour model used in Adobe Illustrator CC:

Hue is represented in degrees from 00 to 3590 corresponding with locations on the circumference of a colour wheel.

Saturation is represented as a percentage where100% denotes a fully saturated hue and 0% denotes a fully desaturated hue.

Brightness is represented as a percentage where 100% denotes a hue at maximum luminance and 0% denotes the darkest possible version of a hue.

## Illuminance

The term Illuminance refers to the quantity of light that falls on a physical surface. Illuminance is typically used to define the usable light supplied by a natural or artificial light source regardless of its total luminosity.

• If we place a book on a table, then different levels of illuminance can be observed when the sky is overcast, under the mid-day sun, in moonlight or artificial light.
• Illuminance is independent of the surface that light falls upon. It is a description of the qualities of the light itself.
• Illuminance is something that can be measured and so is an objective term.
• The implication of this is that a 10-watt light-bulb is a fairly powerful source of light if it is placed next to an observer reading a book. But even a 1000-watt light-bulb will not provide enough light to read by if it is located a hundred metres away.

## Illumination

Illumination or lighting is the deliberate use of light to achieve a practical or aesthetic effect.

Illumination might be provided through the use of artificial light sources like lamps and light fixtures, or natural illumination by capturing daylight.

Daylighting (using windows, skylights, or light shelves) is sometimes used as the main source of light during daytime in buildings.

Specialised forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available.

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

## Incident

Incident light refers to incoming light that is travelling towards an object or medium.

• Incident light refers to light travelling towards something but is often visualised as a single ray.
• A ray diagram (or wave diagram) uses drawing conventions and labels to visualise the path that a single ray (or wave) of light might take as it encounters different media, materials or objects.
• An incident ray (or wave) and its direction of travel is usually shown on a diagram as it propagates through a first medium, prior to encountering a second medium.

https://en.wikipedia.org/wiki/Ray_(optics)#Interaction_with_surfaces

## 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.

## 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.

## Joule

joule is a derived unit of energy in the International System of Units.

• A joule is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre(1 newton/metre or N⋅m).
• It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.
• Where symbol kg is a kilogram, m is a metre, s is a second, N is a newton, Pa is a pascal, W is a watt,  and V is the volt, then the symbol J is for joule.

 Electronvolt prefixes Abbreviation eV to joules millielectronvolt meV 1 eV = 0.001 electronvolt eV 1 eV = 1 eV kiloelectronvolt keV 1x keV =1,000 eV megaelectronvolt MeV 1x MeV = 1,000,000 gigaelectronvolt GeV 1x GeV = 1,000,000,000 1x GeV = 0.00000016021773 teraelectronvolt TeV 1x TeV = 1,000,000,000,000 1x TeV = 1.6021773e-7 petaelectronvolt PeV 1x PeV = 1,000,000,000,000,000 1x PeV = 0.00016021773 exaelectronvolt EeV 1x EeV =1,000,000,000,000,000,000 1x EeV = 0.16021773

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