## 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 compared to a vacuum. The exact speed of light in a medium depends on its optical properties, particularly its refractive index.
• While a vacuum is devoid of matter, in the context of optics, a vacuum is considered a reference point for comparing the speeds of light in other media. It’s not contradictory to refer to a vacuum as a “medium” in this context; rather, it’s a baseline for comparison.
• Within the Earth’s atmosphere, light can travel at speeds near the speed of light. However, when the air is full of water droplets or dust, light travels at significantly lower 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.
###### References
• 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 compared to a vacuum. The exact speed of light in a medium depends on its optical properties, particularly its refractive index.
• While a vacuum is devoid of matter, in the context of optics, a vacuum is considered a reference point for comparing the speeds of light in other media. It’s not contradictory to refer to a vacuum as a “medium” in this context; rather, it’s a baseline for comparison.
• Within the Earth’s atmosphere, light can travel at speeds near the speed of light. However, when the air is full of water droplets or dust, light travels at significantly lower 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.

## 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.

## Fermat’s principle

In the context of refraction, Fermat’s Principle accounts for why light follows the specific path it does when bending at the interface between two media and helps to explain why the bending follows Snell’s Law.

Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.

• In the context of refraction, Fermat’s Principle accounts for why light follows the specific path it does when bending at the interface between two media and helps to explain why the bending follows Snell’s Law.
• Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.
##### Light and Refraction
• Light travels at different speeds in different media. For example, light travels slower in water than in air. When light encounters an interface between two media, it bends due to the change in speed.
• Although Fermat’s Principle doesn’t rely on the concept of conservation of energy, it’s important to note that the total energy of the light wave remains constant before and after refraction.
##### Fermat’s Principle & wavefronts

Fermat’s Principle and refraction are related to the shape of the wavefront at the leading edge of a wave.

• Wavefronts are typically used to describe the behaviour of waves in classical physics, where waves exhibit properties such as interference, diffraction, and refraction.
• While wavefronts are often associated with the behaviour of light, which can exhibit both wave-like and particle-like properties, they are a description of the wave nature of light rather than individual photons.
• At the quantum level, the behaviour of individual photons is described differently, often using concepts such as wave functions in quantum mechanics.
• The shape of a wavefront depends on the light source, the medium through which light is propagating and the obstacles it encounters.
###### References
• In the context of reflection, Fermat’s Principle accounts for why light follows the specific path it does when reflecting off a surface and helps to explain why the angle of incidence is equal to the angle of reflection.
• Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.

## 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.

Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms
Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars
Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles
Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests
Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs
ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks
Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters
Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights
ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests
PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

###### References
• 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.

## 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, a force is anything that can make an object move differently. It’s like a push or a pull that can make an object start moving, stop moving, or change direction. Imagine kicking a soccer ball – the kick is the force that makes the ball move.

• Forces can be either contact forces or non-contact forces.
• Contact forces: These happen when two objects touch, like friction when you rub your hands together, or the push you give the ball. Other examples of contact forces include tension, air resistance and the force exerted by springs.
• Non-contact forces: These act even when objects aren’t touching, like gravity pulling you down, or a magnet attracting a paperclip. Other examples of non-contact forces include gravity, electromagnetism, and the strong nuclear force.
• Forces can make things move faster (accelerate), slower (decelerate), or change direction altogether.
• 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.
##### Push-pull interactions
• Forces can bind objects together or push them apart. 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.
• 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.
##### Fundamental forces
• Four fundamental forces 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.
###### References
• In physics, a force is anything that can make an object move differently. It’s like a push or a pull that can make an object start moving, stop moving, or change direction. Imagine kicking a soccer ball – the kick is the force that makes the ball move.
• Forces can be either contact forces or non-contact forces.
• Contact forces: These happen when two objects touch, like friction when you rub your hands together, or the push you give the ball.
• Non-contact forces: These act even when objects aren’t touching, like gravity pulling you down, or a magnet attracting a paperclip.
• 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 make things move faster (accelerate), slower (decelerate), or change direction altogether.
• 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.

## 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.
• Take light as an example of a force carrier for electromagnetic radiation.
##### Force carriers for the fundamental forces
• Each fundamental force in the universe is transmitted by its own set of particles called force carriers.
• Force carriers act like messengers, mediating the interaction between other particles.
• The force carriers for the four fundamental forces are as follows:
• Electromagnetic Force: The force carrier for electromagnetism is the photon, a massless particle that travels at the speed of light. Photons are responsible for various phenomena like light, electricity, and magnetism.
• Strong Force: The strong force, responsible for binding protons and neutrons within an atomic nucleus, uses gluons as force carriers. Gluons are massless particles that only interact with other particles called quarks, the fundamental building blocks of protons and neutrons.
• Weak Force: The weak force, involved in radioactive decay and some nuclear fusion processes, utilizes W and Z bosons as force carriers. These massive particles are unlike photons and mediate the transformation of one type of subatomic particle into another during weak interactions.
• Gravity: The force that attracts objects with mass, remains a bit of a mystery. Scientists believe a theorized particle called the graviton might be the force carrier for gravity. However, the graviton has not yet been directly detected. It’s predicted to be massless and travel at the speed of light.
###### References
• 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.
• Take light as an example of a force carrier for electromagnetic radiation.
• Light is a form of energy that travels as waves, but it also behaves like a stream of tiny particles called photons.
• These photons are the force carriers for the electromagnetic force, one of the fundamental forces in the universe.
• The electromagnetic force is responsible for a variety of phenomena, including the attraction between oppositely charged particles and the repulsion between like charges.
• Photons can also interact with individual electrons in atoms, causing them to move or change energy levels.

## 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.

## Free electron

A free electron is an electron that is no longer bound to a specific atom and is free to move around.

• Normally, electrons exist in energy levels (orbitals) around an atom’s nucleus. A bound electron needs to absorb enough energy to overcome the attractive force holding it to the nucleus. This energy can come from various sources:

• Heat: Adding heat to a material increases the vibration of atoms, which can bump electrons loose. This is why metals conduct electricity better when heated.
• LightLight, particularly high-energy light like ultraviolet (UV) radiation, can impart enough energy to eject electrons. This is the principle behind the photoelectric effect.
• Electric field: A strong enough electric field can accelerate electrons and provide the energy needed to escape the atom’s attraction. This is important in devices like cathode ray tubes.
• Collisions: In high-energy collisions between atoms or molecules, electrons can be knocked free. This is relevant in processes like particle accelerators.
• Electrons in the outermost energy level (valence electrons) are generally more loosely bound and have a higher probability of being freed by these energy sources.

Free electrons play a role in many electromagnetic phenomena:

• Conduction: In metals, a large number of free electrons move relatively freely throughout the material. This allows them to carry electric current when a potential difference is applied.
• Electricity: The flow of free electrons constitutes electric current in various materials and circuits.
• Chemical Reactions: In some chemical reactions, the transfer of free electrons between atoms or molecules is what defines the reaction.
###### References
• A free electron is an electron that is no longer bound to a specific atom and is free to move around.
• Normally, electrons exist in energy levels (orbitals) around an atom’s nucleus. A bound electron needs to absorb enough energy to overcome the attractive force holding it to the nucleus. This energy can come from various sources:

• Heat: Adding heat to a material increases the vibration of atoms, which can bump electrons loose. This is why metals conduct electricity better when heated.
• Light: Light, particularly high-energy light like ultraviolet (UV) radiation, can impart enough energy to eject electrons. This is the principle behind the photoelectric effect.
• Electric field: A strong enough electric field can accelerate electrons and provide the energy needed to escape the atom’s attraction. This is important in devices like cathode ray tubes.
• Collisions: In high-energy collisions between atoms or molecules, electrons can be knocked free. This is relevant in processes like particle accelerators.
• Electrons in the outermost energy level (valence electrons) are generally more loosely bound and have a higher probability of being freed by these energy sources.

## 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 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 (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.
###### References

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:

##### 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 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.
##### Gama correction & appearance of colours
• The appearance of an image on a digital display is determined by the voltage associated with each pixel:
• For instance, a computer translates the numerical values of each pixel in an image file into a corresponding voltage, which is then transmitted to a monitor. The colour 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.
###### References
• 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.

## Gamut

The term gamut or colour gamut is 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 that is smaller than all the colours that the human eye can see.
• All the colours in an image. Digitizing a photo, changing an image’s colour space, or printing an image onto paper 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.
###### References
• The term gamut or colour gamut is 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 that is smaller than all the colours that the human eye can see.
• All the colours in an image. Digitizing a photo, changing an image’s colour space, or printing an image onto paper 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.

## 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 onto 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 encoding 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.
##### Ganglion cells at work
• 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 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 contextualizing 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.
###### References

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 onto 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 encoding to ultimately converge into a unified representation of the visual world.

## 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!

## Geometric raindrop

A raindrop is often represented as a geometrically perfect sphere. This is an idealized form that rarely exists in the real world. The 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 idealized geometry of raindrops can be applied to every rainbow even though:
• The shape of a real 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 a way to simplify how we think about the behaviour of light. In the case of rainbows, it affects how we think 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.
###### References
• A raindrop is often represented as a geometrically perfect sphere. This is an idealized form that rarely exists in the real world. The 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 idealized geometry of raindrops can be applied to every rainbow even though:
• The shape of a real 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.

## 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.