Fast medium

The speed at which light travels through different media, such as air, glass, or water, is not a constant. Some media are considered “fast” because light passes through them more quickly than others.

  • Light travels through a vacuum at 299,792 kilometres per second.
  • Light travels at slower speeds through other media. However, it’s important to note that referring to a vacuum as a “medium” is contradictory since a vacuum represents space devoid of matter.
  • Within the Earth’s atmosphere, light can travel at speeds near the speed of light. However, in other cases or through different media, light travels at significantly slower speeds.
  • Understanding whether a medium is considered “fast” or “slow” is valuable in predicting the behaviour of light when it crosses the boundary between different media. As such:
    • When light crosses the boundary from a fast medium to a slower medium, it will bend towards the normal.
    • When light crosses the boundary from a slow medium to a faster medium, the light ray will bend away from the normal.
  • In optics, the “normal” is a line drawn in a ray diagram that is perpendicular, or at a right angle (90 degrees), to the boundary between two media.
  • The phenomenon of light bending when it crosses the boundary between different media is known as refraction.
  • The speed of light in a medium is determined by its refractive index, which is a measure of how much the medium slows down light compared to its speed in a vacuum.

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.

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.

Field

An electromagnetic, electric, or magnetic field refers to the region surrounding an object where it can exert a force on another object without direct contact between them.

  • A field can be represented by lines that show the direction of a force experienced by other objects within the field.
  • Fields exist due to the presence of a source object, which generates the field and interacts with other objects within its influence.
  • Electromagnetic fields encompass both electric and magnetic components and are interconnected through electromagnetic waves.
  • Electric fields are associated with both positive and negative electric charges and exert forces on charged objects.
  • Magnetic fields are produced by moving electric charges, such as currents in wires, and can influence the behaviour of magnetic materials and charged particles.
  • According to quantum field theory, all particles and forces in the universe arise from the behaviour of underlying fields, which interact with each other and with particles to give rise to the properties and behaviour of matter and energy.
  • Fields play a fundamental role in many areas of physics, such as electromagnetism, quantum mechanics, and general relativity. They provide a framework for understanding the interactions and forces experienced by objects without direct contact.

Fluorescence

Fluorescence is a type of luminescence, a light source resulting from the temporary absorption and emission of electromagnetic radiation by certain materials. Fluorescence occurs when these materials “catch” light of a specific colour and then quickly “re-emit” it as a different, usually lower-energy (longer wavelength) colour. Unlike light sources that involve flames or extreme heat, fluorescence happens through a rapid physical process in the material itself.

Key features of fluorescence
  • Fluorescence takes place when a substance absorbs light of a specific energy level, gets excited to a higher energy state, and then quickly emits light of a lower energy (longer wavelength) as it returns to its ground state. This emission typically happens within a very short time frame, ranging from nanoseconds to milliseconds. Fluorescence involves:
    • Light absorption: The substance absorbs light of a specific wavelength, exciting an electron within the molecule to a higher energy level.
    • Excited state: The excited electron wants to return to its ground state.
    • Energy emission: Instead of dropping back down, the excited molecule releases some absorbed energy as light of a lower energy (longer wavelength). This difference reflects the “lost” energy used for excitation.
  • Rapid process: This emission happens very quickly, from nanoseconds to milliseconds.
Examples of fluorescence
  • Fluorescent dyes: Used in highlighters, clothing, and biological experiments. These dyes absorb ultraviolet light and emit visible light, making them appear bright.
  • Minerals: Some minerals fluoresce under ultraviolet light, used in identification and dating techniques.
  • Chlorophyll: The green pigment in plants fluoresces under certain wavelengths, contributing to photosynthesis.
Distinguishing fluorescence from bioluminescence
  • Fluorescence differs from bioluminescence as it doesn’t require complex biological reactions. It’s a purely physical process triggered by light absorption.
  • While some bioluminescent systems might exhibit weak fluorescence, the primary light emission mechanism involves enzymatic reactions and doesn’t follow the principles of fluorescence.
The sub-atomic process

The subatomic process involved in fluorescence can be broken down into several key steps:

  • Light Absorption: The process starts with a molecule (the fluorophore) absorbing a photon of light with a specific energy level.
    • This energy excites an electron within the molecule, promoting it from its ground state to a higher energy level (often a singlet excited state).
    • The energy of the absorbed photon and the specific electron transition determine the wavelength of the absorbed light.
  • Internal Relaxation: In some cases, the excited electron might undergo non-radiative transitions within the molecule. This involves losing some energy through processes like vibrations or collisions with other molecules, without emitting light.
    • This internal relaxation typically happens within picoseconds (trillionths of a second) and doesn’t directly contribute to the observed fluorescence.
    • Radiative Emission: Eventually, the excited electron returns to its ground state, releasing energy in the form of a photon.
    • This emitted photon usually has a lower energy (longer wavelength) than the absorbed photon due to the internal energy losses mentioned above.
    • The specific energy difference between the absorbed and emitted light determines the colour of the emitted fluorescence.
  • Excited State Lifetime: The time it takes for the excited electron to emit a photon and return to its ground state is known as the excited state lifetime. This typically ranges from nanoseconds to nanoseconds in fluorescent molecules.
    • A shorter lifetime indicates a faster emission rate, influencing the intensity and overall efficiency of the fluorescence process.
  • Additional details: The specific energy levels involved, electron transitions, and excited state lifetimes depend on the structure and characteristics of the fluorescent molecule.
    • Fluorescence is sensitive to factors like temperature and the surrounding environment, which can affect the internal relaxation processes and emission properties.
    • While the basic principles remain the same, there are different types of fluorescence and specialized fluorophores used in various applications.
Light SourceDescriptionSub-atomic ProcessMechanismVisible LightNaturalArtificial
luminescenceAny process where atoms or molecules emit light. See Bioluminescence, Chemiluminescence, Electroluminescence,
Fluorescence
Electron ExcitationVarious mechanisms involving energy transitions in atoms/moleculesVaries (depends on mechanism)Yes (some mechanisms)Yes (various technologies)
BioluminescenceA form of luminescence:
Light emission by living organisms
Electron Excitation
Chemical reactions initiated and controlled by biological systems within living organisms.YesYesYes
ChemiluminescenceA form of luminescence:
Light emission from chemical reactions
Electron Excitationhemiluminescence relies solely on the chemical energy stored within the reacting molecules.Varies (depends on reaction)Yes
(natural and synthetic)
Yes (glow sticks, analytical tools)
ElectroluminescenceA form of luminescence::
Light emission due to electric fields
Electron ExcitationApplied electric field excites electrons in materialsYesNoYes (LEDs, displays)
FluorescenceA form of luminescence:
Light emission from certain materials after absorbing light
Electron ExcitationTemporary absorption of light, followed by emission of a different (lower energy) color.YesYes
(minerals and plants)
Yes
(dyes, pigments, glow sticks)
Photoluminescence
Light emitting diodeA type of electroluminescence
Semiconductor diode emitting light when current flows
Electron transition
(recombination)
Recombination of electrons and holes in semiconductors releases energy as photonsYesNoYes
Lasers
(Light amplification by stimulated emission of radiation)
A type of photoluminescenceLight amplification by stimulated emission of radiationExcited atoms/molecules release photons, stimulating further photon emission and amplifying lightYesNoYes
Stellar lightNuclear fusionFusion of hydrogen nuclei releases enormous energy, including lightYesYesNo
FireChemiluminescence & Blackbody radiationHot objects emit light (incandescence), and chemical reactions create excited molecules (chemiluminescence)YesYesYes
LightningPlasma processesHot, ionized gas (plasma) emits light through various mechanisms like recombination and BremsstrahlungYesYesNo
Neon signsGas dischargeElectric current excites gas atoms, which emit light upon returning to lower energy levels (similar to fluorescence)YesNoYes
Light bulbs (Incandescent)Blackbody radiationHot filament emits light due to thermal excitation of electronsYesNoYes
SunlampsUltraviolet radiationEmit UV light, causing fluorescence in nearby materialsNo (UV)NoYes
References
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Summary

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.

Force

In physics, force is defined as any interaction that can cause an object to change its velocity, and so to accelerate or slow down. Force is a vector quantity, meaning that it has both magnitude and direction.

  • Forces can be either contact forces or non-contact forces.
    • Contact forces are forces that act when two objects are in contact with each other. Examples of contact forces include friction, tension, and normal force.
    • Non-contact forces are forces that act between objects that are not in contact with each other. Examples of non-contact forces include gravity, electromagnetism, and the strong nuclear force.
  • Forces can bind objects together or push them apart, affecting their state of motion. Force can therefore be described intuitively as a push or a pull with magnitude and direction, making it a vector quantity.
  • Whenever there is a push-pull interaction between two objects, forces are exerted on both. Once the interaction ceases, the forces no longer act, and the momentum of the objects continues unchanged.
  • Objects, bodies, matter, particles, radiation, and space-time are all in motion.
  • On a cosmological-scale, concentrated matter in planets, stars, and galaxies leads to significant push-pull interactions.
  • Motion signifies a change in the position of the elements of a physical system including translational motion, rotational motion, vibrational motion, and oscillatory motion.
  • The push-pull interactions between things are explained by the interplay of forces.
    • The existence of forces explains how objects interact with each other throughout the entirety of the natural world.
    • Forces generate motion and can cause changes in velocity for objects possessing mass.
    • Changes in velocity encompass various scenarios, such as initiating motion from a state of rest, accelerating, or decelerating.
  • There are four fundamental forces that account for all the forms of pulling and pushing between things in the Universe.
    • The electromagnetic force is responsible for interactions between charged particles, such as electrons and protons, and is fundamental to electrical and magnetic phenomena.
    • The weak nuclear force is involved in processes like radioactive decay and plays a role in the interactions of subatomic particles.
    • The strong nuclear force binds atomic nuclei together and is responsible for the stability of matter. It is the strongest of the four fundamental forces, but it has the shortest range.
    • Gravity governs the interactions between massive objects and is responsible for phenomena like planetary motion and the attraction of objects towards Earth.
  • Read more about the four fundamental forces here.

Force carrier

The fundamental forces, along with their corresponding force-carrying particles, serve as the building blocks of nature.

  • Each fundamental force is conveyed by a distinct particle type known as a force carrier. These carriers are responsible for transmitting forces between pairs of particles.
  • To get on board with this, imagine the behaviour of individual subatomic particles, such as electrons or photons, and how they interact with each other in pairs.
  • For example:
  • The concept underpinning force carriers is that everything in the universe is in perpetual motion, akin to billiard balls constantly colliding with one another.
  • The Big Bang initiated the motion of everything and the principle of universal motion asserts that all objects remain in constant motion, driven by the four forces of nature.
  • In an interaction between two electrons that are in motion and collide with one another, the electrons can exchange energy and momentum through the emission and absorption of photons.
  • This can happen in one of two ways:
    • One electron can emit a photon and the other electron can absorb it.
    • One electron can absorb a photon and the other electron can emit it.
  • The amount of energy and momentum that is exchanged depends on the photon’s wavelength. Photons with shorter wavelengths have more energy and momentum than photons with longer wavelengths.
  • For example, if two electrons collide and one electron emits a high-energy photon, the other electron can absorb the photon and gain a lot of energy. This can cause the other electron to move much faster as it is repelled.
  • The force carriers for the four fundamental forces are as follows:
    • The force carrier for electromagnetic force is the photon.
    • The strong force uses gluons as force carriers. Gluons, being massless particles, bind quarks together.
    • The weak force utilizes the W and Z bosons as force carriers. These bosons, possessing mass, interact with the weak force. They play a role in radioactive decay and some aspects of nuclear fusion.
    • The gravitational force. The force carrier for gravity remains elusive. The graviton is a theoretical particle suggested to be the force carrier for gravity. While undetected so far, it is believed to be a massless particle travelling at the speed of light.

Summary

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.

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.

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.

  • Frequency is measured in Hertz (Hz) and signifies the number of wave-cycles per second. Sub-units of Hertz enable measurements involving a higher count of wave-cycles within a single second.
  • The frequency of electromagnetic radiation spans a broad range, from radio waves with low frequencies to gamma rays with high frequencies.
  • The wavelength and frequency of light are closely linked. Specifically, as the wavelength becomes shorter, the frequency increases correspondingly.
  • It is important not to confuse the frequency of a wave with the speed at which the wave travels or the distance it covers.
  • The energy carried by a light wave intensifies as its oscillations increase in number and its wavelength shortens.
Radio waves
  • Radio waves have the lowest frequencies among these types of electromagnetic radiation. They typically range from a few kilohertz (kHz) to hundreds of gigahertz (GHz). Radio waves are commonly used for communication purposes, such as radio broadcasting and wireless communication.
Microwaves
  • Microwaves have frequencies higher than radio waves, typically ranging from several gigahertz (GHz) to hundreds of gigahertz (GHz). They are commonly used in microwave ovens, satellite communication, and radar technology.
Infrared radiation
  • Infrared radiation (IR) has frequencies higher than microwaves, ranging from several hundred gigahertz (GHz) to several hundred terahertz (THz). Infrared radiation is associated with heat and is used in various applications, including thermal imaging, remote controls, and infrared spectroscopy.
Visible light
  • Visible light is the range of frequencies that can be detected by the human eye, approximately ranging from 430 terahertz (THz) for red light to 750 terahertz (THz) for violet light. Visible light enables us to perceive colours and is responsible for our sense of vision.
Ultraviolet
  • Ultraviolet (UV) radiation has frequencies higher than visible light, typically ranging from several hundred terahertz (THz) to several petahertz (PHz). UV radiation is known for its effects on the skin and can be harmful in excessive exposure. It is used in applications like sterilization, fluorescence analysis, and tanning beds.
X-rays
  • X-rays have higher frequencies than UV radiation, typically ranging from several petahertz (PHz) to several exahertz (EHz). X-rays have shorter wavelengths and are commonly used in medical imaging, security screening, and industrial inspections.
Gamma rays
  • Gamma rays have the highest frequencies among these types of electromagnetic radiation, typically exceeding several exahertz (EHz). They have the shortest wavelengths and are associated with high-energy phenomena, such as radioactive decay and nuclear reactions. Gamma rays are used in medical treatments, scientific research, and industrial applications.

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.

Fundamental forces

In physics, fundamental forces cannot be explained through simpler or more elementary interactions, so are regarded as fundamental building blocks of the natural world.

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

Electromagnetic force
Weak Nuclear force
Strong Nuclear force
  • The strong nuclear force binds matter together and is responsible for holding together protons and neutrons which are the subatomic particles within the atomic nucleus. It counteracts repulsive electromagnetic forces that push subatomic particles apart but only operate over the smallest imaginable distances. The strong nuclear force plays a central role in storing the energy that is used in nuclear power and nuclear weapons.
Gravitational force
  • Gravity is the phenomenon that attracts objects with mass or energy towards one another. It affects celestial bodies such as planets, stars, galaxies, and even light. The influence of gravity on smaller objects like human beings in the presence of larger ones, such as planets, is evident. Gravity, such as the Moon’s gravity, leads to ocean tides on Earth. Gravity accounts for the weight of physical objects. Its range is infinite, although its effects weaken as objects move farther apart.
  • Whenever there is a push-pull interaction between two objects, forces are exerted on both of them. Once the interaction ceases, the forces no longer act, and the momentum of the objects continues unchanged in a vacuum.
  • On a macro-scale, concentrated matter in celestial bodies like planets, stars, and galaxies leads to significant push-pull interactions.
  • Everything everywhere is in motion. Nothing in the whole Universe is stationary unless its temperature is reduced to absolute zero. In reality, nothing can be cooled to exactly absolute zero.
  • Objects, bodies, matter, particles, radiation, and space-time are all in motion. The concept of motion also applies to the movement of images, shapes, and boundaries.
  • Motion signifies a change in the position of the elements of a physical system including translational motion, rotational motion, vibrational motion, and oscillatory motion.

Gamma correction

Gamma correction, also referred to as gamma encoding, is an image processing technique that adjusts the brightness and contrast of an image to achieve a more natural and visually pleasing appearance.

  • Gamma correction of digital images prevents excessive storage of information about highlights that are invisible to humans and ensures sufficient information is retained for 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 (e.g., JPG or TIFF) and its corresponding brightness when displayed on-screen.
  • Gamma correction is typically performed to compensate for the non-linear relationship between the input signal and the displayed brightness on a monitor or screen.
  • In the case of a black-and-white image, a gamma function impacts highlights (brightest values), mid-tones (greyscale), and shadows (dark areas) in distinct ways.
  • Gamma correction is not limited to black and white images but applies to colour images, where it affects colour balance and contrast.
  • The appearance of an image on a digital display is determined by the voltage associated with each pixel:
    • For instance, a computer utilized to display black-and-white images translates the numerical values of each pixel in an image file into a corresponding voltage, which is then transmitted to a monitor. The brightness of a pixel increases with higher voltages.
    • The ideal relationship between stored values and appearance is non-linear, meaning that a voltage change does not directly result in a satisfactory brightness change from an observer’s perspective.
    • For many TVs and computer displays, doubling the voltage of a specific pixel will not make it appear twice as bright. Therefore, gamma correction selectively adjusts voltages to enhance the overall appearance.
  • Gamma correction can help achieve accurate representation of images across various display devices and ensure consistent visual experiences.
  • Different applications and devices may have different default gamma settings, and users can often customize these settings based on their preferences.

Gamut

The term gamut or colour gamut can be used to describe:

  • The range of colours that a specific device or system can display or reproduce.
  • The range of colours that the human eye can see in specific conditions.
  • A range of colours smaller than all the colours that the human eye can see.
  • All the colours in an image. Digitising a photo, changing an image’s colour space, or printing it might change its gamut.
  • The range of perceived colours (visible to a human observer) is always greater than the range that can be reproduced by any digital device such as a screen, monitor or projector.
  • Digital cameras, scanners, monitors, and printers all have limits to the range of colours they can capture, save, and reproduce.
  • The main use of digital colour spaces and colour profiles is to set the gamut of colours that can be used to accurately reproduce or optimise the appearance of an image.
  • It is currently impossible to make a digital device that can reproduce the same range of colours that the human eye can see.

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!

Ganglion cells

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 types of neurons (interneurons): 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 to 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 eye’s 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 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:
    • The information must feed into and support the distinct attributes of visual perception and be available to be resolved within the composition of our immediate present visual impressions whenever needed.
    • The 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.
    • The 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 trivariate responses to meet the criteria of efficient colour information complete with all the necessary contextualising detail ready for transmission. We can assume it is in response to these demands that every stimulus to the eye can be accurately and objectively defined in both space and time in ways relevant to everyday circumstances.

Geometric raindrop

In idealised terms, a raindrop is often represented as a geometrically perfect sphere. This simplification aids in comprehending the physics of rainbows, even though real-life raindrops seldom maintain such perfect spherical forms.

  • The understanding derived from studying the idealised geometry of raindrops can be applied to every rainbow despite the fact that:
    • The shape of a raindrop is highly variable and depends on factors including size, speed of descent, and turbulence.
    • Each rainbow observed in our daily life and the arrangement of droplets within it is unique due both to chance and to a wide range of environmental factors.
    • By way of summary, the form of a rainbow and the arrangement of raindrops within it depends on a variety of unique and changing conditions. These include the size, shape, and arrangement of the raindrops that make up the rainbow, as well as the position of the sun, the observer’s location, the clarity and composition of the atmosphere, and the presence of any other light sources or reflective surfaces. So, each rainbow that we observe is unique, shaped by both random variations and a wide array of environmental factors.
  • The idea of light rays is also a way to simplify the way we think about the behaviour of light as it approaches, passes through and exits raindrops towards an observer.
    • In reality, the notion of light rays does not describe an inherent physical property of light; rather, it’s a simplification for illustrative purposes.
    • More precise descriptions of light refer to it as composed of particles called photons, or as exhibiting wave-like properties.

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.

Geometrical optics

About geometrical optics
  • Geometrical optics, also known as ray optics, is one of the two main branches of optics, the other being physical optics.
  • Geometrical optics is based on the assumption that light travels as a straight line and is useful in explaining various optical phenomena, including reflection and refraction, in simple terms.
  • Geometrical optics is a useful tool in analyzing the behaviour of optical systems, including the image-forming process and the appearance of aberrations in systems containing lenses and prisms.
  • The underlying assumptions of geometrical optics include that light rays:
    • Propagate in straight-line paths when they travel in a uniform medium.
    • Bend, and in particular, refract, at the interface between two dissimilar media.
    • Follow curved paths due to the varying refractive index of the medium.
    • May be absorbed as photons and transferred to the atoms or molecules of the absorbing material, causing the absorbing material to heat up or emit radiation of its own.

Gravitational force

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

  • Gravity is the phenomenon that attracts objects with mass or energy towards one another.
  • It affects celestial bodies such as planets, stars, galaxies, and even light.
  • The influence of gravity on smaller objects like human beings in the presence of larger ones, such as planets, is evident.
  • Gravity, such as the Moon’s gravity, leads to ocean tides on Earth.
  • Gravity accounts for the weight of physical objects. Its range is infinite, although its effects weaken as objects move farther apart
  • Gravitational force is a universal force, meaning that it acts between all objects with mass, regardless of their composition or charge.
  • Gravitational force is a long-range force, meaning that it can act between objects that are very far apart.
  • Einstein’s theory of special relativity showed that mass and energy are equivalent, and can be converted into each other. This is expressed in the famous equation E = mc2, where E is energy, m is mass, and c is the speed of light.
  • This means that any object with energy also has mass, and therefore can be attracted by gravity. For example, light has energy, and therefore has mass. This is why light can be bent by very large objects such as galaxies.

Summary